If you're one of those fans trying to figure out why Anthony Barr isn't playing like, well, Anthony Barr of late, you're not alone.

Vikings coach Mike Zimmer was asked Monday, "What have you thought of Anthony Barr the last few weeks?"

"Uh," Zimmer began, "He can do a lot better."

Pause. Reporter: "What areas do you think he could do better at?"

"All of them," Zimmer said.

Zimmer didn't single out his third-year linebacker, or any other player for that matter, the day after a 34-6 loss to the Colts at U.S. Bank Stadium. He wasn't impressed by any of them.

Barr

"I think for some reason they were maybe lethargic is the right word," Zimmer said. "I kind of felt it in pregame, and I don't know the reason."

Barr played all 70 defensive snaps. He finished with one assisted tackle. That could change after coaches review the film, but it's not expected to go up much as Barr turned in another performance that didn't stand out and seemed to lack energy.

The irony is Barr is the healthiest he has been as an NFL player. Injuries caused him to miss two games as a rookie and three games a year ago. Both times, when he went down because of injury, he was playing at a high level. This year, he has played in every game and has not been designated as questionable or doubtful on an injury report.

D-lineman Johnson tears hamstring

Tom Johnson tore a hamstring Sunday and will be placed on season-ending injured reserve, according to a USA Today report.

Johnson, a pass-rushing reserve defensive tackle, has recorded 17 tackles and two sacks in 14 games this season. He also has one interception.

With Johnson headed to IR, the Vikings have only two healthy defensive tackles on their active roster. They could bring back Toby Johnson, who was waived over the weekend to make room for running back Adrian Peterson, to join Linval Joseph and Shamar Stephen in the defensive tackle rotation.

Mental errors persist

There were four mental blunders from Sunday's game that were particularly aggravating for Zimmer: the false starts by receivers Charles Johnson and Stefon Diggs, the neutral zone infraction on defensive end Everson Griffen on third-and-5, and Linval Joseph contacting the long snapper while toppling over him, a personal foul that took the Colts' 21-yard field goal off the board and replaced it with a 1-yard touchdown run two snaps later.

"It was pretty poor," Zimmer said. "That's kind of been a deal that we've been dealing with all year long. I don't know how to observe a mental error. But I know we're making these mistakes. Really, all four of those, we can't do it. But it's been rearing its ugly head for a long time and I can't get it fixed."

Not much to see of AP

Asked how Peterson looked in his first outing since missing 11 weeks because of a torn meniscus, Zimmer said, "Good. There were some runs he needed to do a little bit better. There were some times there wasn't much there."

Peterson's eighth snap and fourth carry of the game was a red-zone fumble that the Colts turned into a touchdown and a 24-0 second-quarter lead. So Peterson wasn't used much after that point.

"He only had 12 plays," Zimmer said.

Pass rush quiet

The Vikings didn't register a sack for the first time since the Eagles game Oct. 23.

"[The Colts] never really gave us opportunities to rush the quarterback," Zimmer said. "We didn't stop the run, so that made it difficult. And then, like the [50-yard] touchdown pass they threw to [Phillip Dorsett], they blocked nine guys on the play. And on the third downs, they were chipping with the tight ends the majority of times.

"But we didn't do a good job in coverage either. All that stuff goes hand in hand."

Staff writer Matt Vensel contributed to this report.