A quarterback, like Cody Kessler, is sometimes a runner and sometimes a passer. (Bruce Kluckhohn / Associated Press)

Sometimes a quarterback, at least one not named Tom Brady, is a runner and sometimes he’s a passer. Under a new rule this season, some defensive penalties will hinge on how officials make that determination.

While most of the early talk during the NFL’s preseason has concerned implementation of the helmet rule, this rule — call it the Anthony Barr rule or, more precisely, the Aaron Rodgers rule — came up Saturday and the Minnesota Vikings were again involved.

This time, the key players were Antwione Williams and Cody Kessler of the Jacksonville Jaguars. Williams was flagged for what looked like a perfectly legal sack of the quarterback, if you were living in 2017. Head to the side? Check. Quarterback brought down cleanly with both players emerging uninjured? Check.

The problem was that Williams’s weight landed on Kessler and he was called for roughing the passer.

Vikings Coach Mike Zimmer hated the fourth-quarter call as he stood on the sideline, but he later changed his tune. After all, he has a franchise quarterback, too.

“After I calmed down a little bit, I looked at it and he was, his head was to the side and he was going to the side, and if he would have just rolled, but he kind of pumped him to the turf,’’ Zimmer said (via the Pioneer Press). “I actually think that was a good call.’’

Observers will recall that Rodgers’s 2017 was essentially ended in Week 6 when Barr brought him down, with his weight driving Rodgers, who was out of the pocket on the play, into the ground and snapping his right collarbone. Barr was not flagged on the play and received death threats because of it.

“It was deemed a legal hit,” Rodgers, who required surgery to repair the displaced fracture and played in only one game the rest of the season, said last December. “You know that you don’t have the same protection outside the pocket. A simple shove-down probably would have sufficed in that situation, but it is what it is.”

Had that play occurred in 2018, it would have drawn a flag.

“Players will have to kind of roll to the side when they make that tackle instead of plopping down on them,” referee Pete Morelli said earlier this month (via the Pioneer Press). “So, yeah, [the] Aaron Rodgers [tackle] would be a foul this year. As long as he’s out of pocket, established, and all that. If he’s running, that’s not the same.’’

Even though Rodgers was out of the pocket, Morelli said he had reestablished himself as a passer.

“When he’s a passer in a defenseless position,” Morelli said. “If you roll out and get set up, you’re still a passer now. But if you’re rolling out and throwing, and a guy is chasing you and tackles you, then you’re not defenseless. They get two steps and they can tackle you. Becoming defenseless is setting up [for a pass] again, pocket or outside the pocket.”

Barr admitted that it is a “very difficult situation” to be in. “You’re playing fast, trying to make a play on the ball. It’s going to be tough,” he said in early August. “It will be interesting to see how that’s . . . called this year.

“I’m sure there will be some debate from the players, from the coaches, from the officials as to who’s right and who’s wrong. … You can complain about it all you want, but it’s not going to change unless they change it. It’s your duty as a player to continue to play by the rules.’’

The 2017 season was all about which quarterbacks were injured and weren’t playing.

In addition to Rodgers, the two-time NFL MVP, the list of star players who were sidelined by injuries included Philadelphia’s Carson Wentz, Houston’s Deshaun Watson and Miami’s Ryan Tannehill in addition to Indianapolis’s Andrew Luck. Those injuries wouldn’t have been prevented by the rule change, but clearly teams know it’s in their best interests to protect the quarterback.

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