EDEN PRAIRIE, Minn. -- Adrian Peterson said his first two days on the practice field went "pretty smoothly," after the Minnesota Vikings' decision to designate him for return from injured reserve. But coach Mike Zimmer cautioned the Vikings still have plenty to determine before deciding when Peterson would play and how he would be used.

"We'll have to see where he's at in conditioning. There's so many things to figure out," Zimmer said. "I think he's in pretty good shape, but he hasn't sat in meetings for three months. There's a lot of things to consider."

Peterson, who tore the meniscus in his right knee against the Green Bay Packers on Sept. 18, practiced for the second time on Thursday. The running back has said he's shooting to return for the Vikings' game against the Packers on Dec. 24, but he has also cautioned he wouldn't play if the Vikings aren't in the playoff race, adding it would be "pointless" to return for games that didn't have postseason implications.

Asked on Thursday about the running back saying it will be his decision when he returns, Zimmer said, "I wouldn't say it was his decision. I would say it's a combination. We'll sit down and talk, and the medical people [will weigh in]. It's a lot to do with the medical."

Said Peterson on Thursday about the Packers game: "That was the goal I had set for [myself], but I'm going to continue to take it one day at a time. When that time comes, as a group, we'll sit down and talk."

The 31-year-old running back will have to weigh his future in deciding whether playing this season is worth it. The Vikings hold a team option on Peterson for 2017, with a $6 million roster bonus due in March and $18 million cap hit, but if Peterson were to return to Minnesota next season, it would seem likely to be on a restructured contract.

Vikings coach Mike Zimmer on when RB Adrian Peterson could possibly play: "We'll have to see where he's at in conditioning. There's so many things to figure out. ... There's a lot of things to consider." Andy Clayton-King/AP Photo

"I'd be lying to you to say that [the future] is not factoring in [to the decision]," he said. "In my mind, I'm telling myself that it's 100 percent healed, but I really won't know until I look and see it from images. So with that, you've got to be smart about how you approach everything.

"It's not really [about proving myself to the Vikings]. A little bit, but I feel like thus far, I've proven myself. I've been doing this for 10 years. It's not the first injury I've overcame, came back and was still able to be successful. Proving anything to the organization, I don't think I have to do that. I think it speaks for itself."

Peterson has been wearing a knee brace in practice the last two days, and though he has typically shied away from using one in games, he said he'd likely wear one the rest of this year, as he tries to come back from an operation that can keep players out between four and six months.

"It's like, 'Why not?' The new titanium braces are superlight," he said. "It's a little uncomfortable, because I really don't like anything on my knees. But taking in the situation, I'll wear it."

The Vikings' tumultuous season means Peterson would be returning to a team that looks noticeably different than the one he played for on Sept. 18. Since Pat Shurmur replaced Norv Turner as the offensive coordinator on Nov. 2, the Vikings have been in shotgun sets two-thirds of the time, with their 247 shotgun snaps ranking seventh in the league in that time, according to ESPN Stats & Information.

Peterson said Thursday that some of the Vikings' offensive terminology has changed in the switch from Turner to Shurmur, though the core of the offense remains the same. "There are a couple wrinkles in there that are a little different," he said. "The meeting room is different as well. We have the same offense we had previously, but just with some new twists."

Zimmer added he has not yet talked with Peterson about how the Vikings plan to structure his workload. "That's completely up to the medical people what he does," he said.

A Peterson return should provide a boost to a Vikings rushing attack that's on pace for the worst rushing average since the New England Patriots in 1994. Peterson had only 50 yards on 31 carries in his first two games, but Zimmer on Thursday attributed many of the issues to the Vikings' run blocking and the first two regular-season games in a new system under offensive line coach Tony Sparano.

Asked if Peterson could help even if he's less than 100 percent, Zimmer said, "Well, I think people will play us differently. I'm not saying he's less than 100 percent."