Standing on the sideline Sunday afternoon, when the opposing team has the ball, Tom Brady will watch a quarterback in possession of the things he had for so long. Patrick Mahomes plays for a coach who tailored an offensive system to maximize his powers. He operates with complete assurance in his talent and the players around him. Brady will see a quarterback in full control — of his performance, of his team, of the entire league.

This week’s showdown between Brady’s Tampa Bay Buccaneers and Mahomes’s Kansas City Chiefs provides a stage for the quarterback who defined the previous era of the NFL and the quarterback who will define the next. Their meeting occurs on terms contradictory to their age and status. Mahomes, 25, is the most secure force in the NFL, in perfect alignment with his franchise as he stands astride the league. Brady, 43, is struggling to find his footing in a new place, still grasping amid unfamiliar uncertainty. Mahomes radiates. Brady gnashes.

When Brady chose the Buccaneers this offseason, the appeal was evident. Warm weather. Great wide receivers. Tompa Bay, here we come. The rigors of the season, of trying to learn a new scheme after two decades with the New England Patriots doing something else, have exposed the downsides of leaving Bill Belichick and starting over in middle age, especially during a season with irregular and limited preparation.

Brady has led the Bucs to a 7-4 record and planted them firmly in position to reach the playoffs. But he returned for a 21st season to win the Super Bowl, and with five regular season games remaining, Tampa Bay is far afield. The division rival New Orleans Saints have twice clobbered the Bucs. He has been inaccurate throwing deep. He has struggled against pressure. His coach, Bruce Arians, hurls public criticism at him that is gentle but oddly persistent.

“Offensively, it’s just a matter of each and every week if the quarterback plays well or not,” Arians said Tuesday when asked during his team-owned coach’s show about the Bucs’ inconsistency. “And our job is to make sure he’s comfortable and let him play well.”

In the spring, commentators identified adjusting to Arians’s system, especially without a normal offseason, as a major hurdle for Brady. Those kinds of analyses often ring simplistic and hollow, but this one turned out to be exactly right. Arians employs a scheme reliant on deep passing, a contrast to the quick, precise throws Brady made an all-time-great career on in New England.

“I don’t think it’s a confidence problem whatsoever,” Arians said after Monday’s 27-24 loss to the Los Angeles Rams. “It’s not lack of trust. It’s just lack of continuity within the offense of the whole picture.”

Said Brady: “I have to do a better job. Absolutely.”

The kind of offense Brady is running matters, but more important may be how he learned. Brady was able to communicate in a common language with Patriots coaches he had worked with for years. In the Super Bowl two years ago, New England summoned a set of plays it had used years earlier at a crucial juncture, even though it had not discussed or practiced them.

Now Brady is playing catch-up. In New England, he was on autopilot. In Tampa Bay, he’s still trying to figure out the controls.

“The lack of practice time and learning everything from spring through camp is still showing up,” Arians said at a news conference Tuesday. “It’s so much easier when you have all that practice time. When you have 300 or 400 reps with the guys, it’s a big difference than having 20 or 30 practice reps.”

One former AFC executive who competed against Brady for years said Brady’s age has affected his skills this season but not to great effect. “It’s still there,” said the executive, who spoke on the condition of anonymity to give a candid evaluation. “I don’t think it’s where it once was, but it’s not far away, either.”

The executive agreed with Arians’s assessment. Even though the Bucs have played 11 games, the lack of offseason work with a new quarterback still surfaces.

“It’s definitely a work in progress,” he said. “This is a guy that’s been in one system for a long time. We say it all the time, but we take for granted no preseason games, no [organized team activities in the offseason]. People don’t make enough of that, in my opinion.”

Arians insisted Brady can — and has — thrived in his system. He lets Brady choose plays from offensive coordinator Byron Leftwich’s game plan on the sidelines during games. “He can do anything,” Arians said. “It’s not like he can’t do it. I’ve seen him do it all the time. He was doing it in September. He hasn’t changed.”

Brady’s performance this season has suggested that Arians is trying to fit Brady into a system ill-suited for him. Brady has attempted 58 passes of more than 20 yards in the air, according to Pro Football Focus, tied with Green Bay’s Aaron Rodgers for most in the NFL. Three of his eight interceptions have come on them. His passer rating when throwing deep is 68.7, 29th in the league. Even when crediting him for the three deep throws his receivers dropped, Brady has thrown 21 accurately, a percentage that ranks him 29th.

On Monday night, Brady attempted nine passes that sailed at least 15 yards downfield, according to NFL NextGen Stats. He completed one of them and threw two interceptions, both thrown well off the intended target and directly into rookie safety Jordan Fuller’s arms. Insisting Brady has the ability to throw deep, Arians placed blame on his future Hall of Fame quarterback.

“We’ve gotten the guys open,” Arians said. “We’ve just missed them. There are times when coverage dictates you go to that guy. I think we can do a better job of utilizing the deep ball in our game plan. When they’re there, we need to hit them. We can’t have them going off our fingertips, and we can’t overthrow them.”

Arians’s comments continued a pattern. He is not shy about letting reporters and fans know about Brady’s shortcomings. It is refreshing for outside parties to hear Arians’s honest assessment, but it’s easy to wonder what Brady thinks. Belichick could be lacerating about Brady’s performance, but he saved his rebukes for the practice field or film room. Arians is blunt on the news conference lectern.

Arians, when asked Monday night about dropped passes by Bucs running backs: “A couple of them were not really good throws, either.”

On Brady’s game-sealing interception: “Just a misread of the coverage.”

Brady agreed with Arians’s diagnosis. On the Bucs’ final play, Brady saw tight end Cameron Brate sprint behind an underneath defensive back and decided to throw it to him down the seam. At the last second, he saw Fuller racing behind Brate. Brady still threw the ball — too long for Brate and directly into Fuller’s waiting arms.

“It was just a bad read, bad throw, decision, everything,” Brady said. “It can’t happen.”

The former executive cited the play as evidence of Brady’s confusion — the Rams were playing a version of cover-one, with one safety playing deep. Any quarterback would know never to make that throw, let alone an all-time great.

Part of it may be explained by how easily the Bucs come undone when the pass rush works against them. Brady’s passer rating when pressured is 55.0, which is 26th in the NFL. Part of it may also be how Brady has adjusted slowly to a new home. The Bucs have one of the best receiving corps in the league with Mike Evans, Chris Godwin, Antonio Brown and Rob Gronkowski, plus capable running backs Ronald Jones II and Leonard Fournette. When it all clicks, it looks great. More often, the Bucs play as less than the sum of their parts.

“[When] you don’t have a spring at all or a real training camp, it’s hard on the quarterback, especially when you’ve done something for 20 years and then throw all these guys at him,” Arians said. “But I don’t think you can have too many good players. Looking at the team coming in here this week, I don’t think they have a problem with having too many really good players.”

There, then, is another separator between Brady and Mahomes, his counterpart Sunday. The Chiefs built a team around Mahomes. The Bucs dropped Brady into the middle of a collection of players. It has made a difference. It explains why one quarterback is so fully at ease and why one quarterback who has accomplished so much can still be trying to figure out what comes next.