Disappointed but optimistic, Lions general manager Martin Mayhew knows starting running back Jahvid Best will be back on the football field. He just doesn't know when.

The sluggish progress of his recovery from a concussion last October has everyone in the organization feeling anxious, among other emotions.

"A little bit surprised, a little bit disappointed," Mayhew said Tuesday before the Lions' training camp practice. "Everything was trending toward him being ready to go on the first day of training camp. It just didn't happen that way. We have to kind of adjust. We have adjusted our expectation on that."

Best wasn't cleared and hasn't participated in full team workouts since he suffered a concussion in Week 6 against the 49ers last season, his second in 2011 and third in his career. He said in June that concussion-like symptoms were gone and he was ramping up his training.

Best was cleared for football activity, but coach Jim Schwartz said Thursday that Best hasn't been cleared for contact.

Mikel Leshoure, a second-round pick in 2011 who missed last season while recovering from a torn Achilles suffered in August, passed his physical and is likely to work with the first-team offense along with Kevin Smith. But Leshoure was slowed Monday by a hamstring injury, leaving the Lions to wonder if their running back woes have returned.

They'll practice patience with Best.

"It's really frustrating," Mayhew said. "Like Jim said, it's not like a hamstring."

Schwartz said Best received input from experts around the country but continues to work with the Lions' doctors. Mayhew said on a conference call to update Best's condition "there are 20 guys on the call. He's got the experts, and those guys will make a determination when he's ready to play."

Best missed the final 11 games last season, including the first-round playoff loss at New Orleans.