Injured Indianapolis Colts quarterback Peyton Manning and head coach Jim Caldwell watch from the sidelines in the first half of their game against the Cincinnati Bengals last month. (Tom Uhlman/ASSOCIATED PRESS)

Peyton Manning is the only four-time most valuable player in NFL history. Yet his worth to the Indianapolis Colts never has been more evident than this season, when the team’s standout quarterback has been relegated to the sideline after his second neck surgery of the year.

Without Manning, the Colts have been transformed from perpetual championship contender to league also-ran, unglued not only on offense but also on defense. They’ll take an 0-8 record into Sunday’s game at home against the Atlanta Falcons.

After nine straight seasons in which they had at least 10 victories and reached the playoffs, they have been reduced this year to trying to avoid the ignominy of a winless season and pondering what they might do in April if they have the top overall selection in the NFL draft — and first shot at Stanford star quarterback Andrew Luck.

The total unraveling of the Colts’ season leaves little doubt, experts say, that Manning is among the most indispensable players the sport ever has seen.

“It’s like taking the mom out of the household,” said former Green Bay wide receiver Antonio Freeman, who played with another centerpiece quarterback, Brett Favre, with the Packers. “There’s no way to replace Mom.

“There wasn’t a lot there before Manning and there’s not a lot there now that he’s hurt,” Freeman added. “People in Indy, all they can do now is think about the good days when he was in there playing.”

‘They lost their leader’

There are other examples of teams experiencing major dropoffs in performance after losing a top quarterback. The Denver Broncos went 6-10 in the 1999 season, a year after John Elway led the franchise to its second straight Super Bowl triumph and then retired. The Buffalo Bills went 6-10 in the ’97 season after going 10-6 and reaching the playoffs in ’96, their final season with Jim Kelly, the quarterback who led them to four consecutive Super Bowl appearances.

But neither of those teams fell quite this far, and former Bills coach Marv Levy said there were other factors in the decline in Buffalo.

Backup quarterback “Frank Reich had already moved on. We had a young quarterback in Todd Collins. But we had also just lost people like Cornelius Bennett and Kent Hull, who had been key members of those teams,” Levy said. “Certainly losing Kelly was a big cut and part of the reason why we didn’t play at the same level, but it wasn’t the only reason. This is unprecedented, to my knowledge, for this to happen to a team with the loss of one player.”

Another prominent former NFL coach, Dick Vermeil, said he thought when the season began that the Colts could be a “close to break-even” team minus Manning.

“I started in the league in 1969 and I can’t ever remember this happening,” Vermeil said. “I don’t know if it’s 100 percent because of him being out or if they were in decline already. But they certainly wouldn’t be this bad if he was playing.”

Other teams have weathered the loss of a franchise quarterback much better. The New England Patriots, for instance, went 11-5 in the 2008 season with Matt Cassel at quarterback after Tom Brady suffered a season-ending knee injury in the opening game, leading to the conclusion that the Colts depend on Manning even more heavily than the Patriots rely on Brady, their three-time Super Bowl winner.

“They not only lost their quarterback, they lost their leader,” said Vermeil, the former coach of the Philadelphia Eagles, St. Louis Rams and Kansas City Chiefs. “They lost all their offensive coordination. He was deeply involved in everything they did, from preparation to execution during the game.”

The Colts lured veteran quarterback Kerry Collins out of retirement before the season. But Collins made only three starts before suffering a concussion that landed him on the season-ending injured reserve list. The Colts next turned to Curtis Painter at quarterback. He is the league’s 28th-rated passer. The Colts rank 30th in the league in total offense (based on yards gained) and 30th in scoring offense.

“Their running game worked off their ability to pass the ball,” Freeman said. “He [Manning] would stand there at the line of scrimmage before the snap and point everywhere. As annoying as it was to watch, he was pointing things out to his offensive linemen and making sure everyone was on the same page with everything. He controlled the football game. I knew they’d be a bad team without him, but not this bad.”

A possible return

Levy wondered whether other losses have contributed to the Colts’ rapid decline. The Colts also have lost two defensive starters, linebacker Gary Brackett and safety Melvin Bullitt. But many observers believe the only difference that matters is Manning’s absence.

The Colts rank 31st in the league in total defense and last in scoring defense. In previous seasons, the Colts were at their best when Manning built them a lead and defensive ends Dwight Freeney and Robert Mathis could rev up the pass rush against opposing offenses trying to catch up. Now that formula has been disrupted.

“That was a pretty good defense, and now it’s not playing well at all,” Freeman said. “The defense is taking far more snaps now because the offense is going three [plays] and out. Manning controlled the pace of the game.”

The Colts have not placed Manning on injured reserve, leaving open the possibility that he could practice with the team or even play in a game late this season. Manning spoke to reporters last week and said he would like to practice or play next month if he is physically able and is cleared by doctors.

“All I know is I like playing,” Manning said. “I enjoy being out there. I miss being out there in the huddle. When I’m on that sideline looking at the huddle, even though I’m probably just 25 yards away from the huddle, it feels like I’m a million miles away.”

The Colts and their fans likely have had the same feeling all season. Until this season, Manning hadn’t missed a start since the Colts made him the top overall pick in the 1998 NFL draft.

Now the Colts face an intriguing decision if they end up with the top choice again: Should they take Luck? Even if Manning, who is in the first season of a five-year, $90 million contract, makes a successful return, he will turn 36 in March.

They would have to decide whether to retain Manning by February, when they would owe him a $28 million option bonus. If the Colts decline, a possibility many in the league consider remote, Manning would become an unrestricted free agent.

Vermeil said he thinks the Colts should draft Luck if given the chance. He likened it to the Packers selecting Aaron Rodgers in the opening round of the 2005 draft with Favre still in the fold.

That move made for some uncomfortable moments in Green Bay, ending with Favre’s bitter departure in the summer of 2008. But Rodgers, now in his fourth season as the starter, has become one of the league’s best quarterbacks. He took the Packers to a Super Bowl victory last season and has led them to a 7-0 record this season.

“I think I would do it,” Vermeil said. “We’re all watching Aaron Rodgers in Green Bay and how he’s playing.”