"I'm a little bit surprised, a little bit disappointed," Mayhew admitted. "Everything was trending toward him being ready to go the first day of training camp."
Best had a season-ending concussion during Week 6 against the San Francisco 49ers -- 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 that he's being held out by doctors despite not having any obvious concussion symptoms.
"It's definitely frustrating," he said.
Best rushed the ball 84 times in six games last season for 390 yards and two touchdowns. He also had 27 receptions for 287 yards and one touchdown.
The former first-round draft pick is known as an explosive and nimble runner. In limited action last season, Best was able to break off an 88-yard run and a 60-yard catch.
The Associated Press contributed to this report.