"I'm a little bit surprised, a little bit disappointed," Mayhew acknowledged. "Everything was trending toward him being ready to go the first day of training camp."
Best had a season-ending concussion on Oct. 16, 2011 -- his second of the year and at least his third overall -- and seemed to be making progress during the offseason.
Like the Lions, Best isn't pleased he's being held out by doctors despite not having any obvious concussion symptoms.
"It's definitely frustrating," he said.
Detroit is counting on Best to help strike a balance with its stellar passing game led by quarterback Matthew Stafford and receiver Calvin Johnson.
The Lions also are banking on defensive end Cliff Avril reporting to camp to sign a $10.6 million, one-year deal after he and the team couldn't agree on a long-term contract.
"Eventually, I know he will be here," Mayhew said. "And he'll be in great condition when he gets here."
Messages left with Avril, seeking the planned date of his arrival in Allen Park, haven't been returned.
"If anything is positive or encouraging to me, he's very optimistic about how he's going to perform this season," Mayhew said. "He has his reasons for not being here. We made a very solid offer to him that we felt was a really good offer. He was willing to bet on himself and bet that he's going to outperform what was on the table, and I like that."
Information from The Associated Press was used in this report.