"I think just in terms of everything that's been going around with concussions, everybody's being extra careful," Best said. "If this was four or five years ago, I probably would have played last season. Now that everything's under a thin microscope, it takes a lot longer."
Best missed more than half of last season after a concussion against San Francisco on Oct. 16. He also had concussion problems last preseason and when he was a college player at California.
The Lions opened practice Friday, and Best was put on the active physically unable to perform list.
"He has made tremendous strides in a lot of areas, including being able to work out with the rest of the team and lift hard and do all those things. But there's still some areas that he's got some ground to cover," coach Jim Schwartz said. "We're very optimistic we'll get him back, but his progress is really not judged day to day. It's going to be judged week to week."
The Lions practiced indoors because of a rainstorm in the Detroit area. Best's lingering injury problem was the day's big story, and the team was also without defensive end Cliff Avril, whose absence wasn't much of a surprise. The Lions retained Avril with the franchise tag but failed to reach an agreement with him on a long-term deal.
Defensive end Lawrence Jackson practiced with the first team.
Best wouldn't go into specifics about what needs to happen for him to return.
"I'm not going to get into it. I'm just working as hard as I can to get back out there, and as soon as I get cleared I'll be back out there," he said. "I've talked to thousands of doctors. Not literally, but I've talked to a bunch of doctors."
Best said he's not concerned his career could be in jeopardy.
"That has not crossed my mind yet," he said. "Right now I'm just waiting to be cleared."
Best worked out with teammates during offseason activities, but now the season is approaching. He started nine games for the Lions as a rookie in 2010 and rushed for 390 yards on 84 carries last season. Without Best, Detroit needed to piece together some semblance of a running game to complement quarterback Matthew Stafford and star receiver Calvin Johnson.
Running back Mikel Leshoure missed all of his rookie season last year with a torn left Achilles tendon, and he'll miss the first two games this season without pay and will have to give up two more game checks for violating the NFL's substance-abuse policy.
That leaves Kevin Smith, who ran for 356 yards last season on 72 carries, although he has an injury history as well.
Kick returner Stefan Logan could also see some more time at running back.
Schwartz says the team has plenty of experience playing without Best, and the young running back won't be rushed back.
"Times have changed in the NFL," Schwartz said. "There's no more being unconscious and going back and playing in a game, or having a concussion and coming back too soon. I think that's a good thing."