Shanahan made the announcement during his press conference on Monday.
Griffin, the NFL's Offensive Rookie of the Year last season, had been cleared to play by surgeon Dr. James Andrews after having reconstructive right knee surgery to repair a torn ACL and torn LCL in January. Shanahan said Andrews had "a couple of concerns" about Griffin, but did not reveal the specific nature of any issues.
"I feel very good where Robert's at," Shanahan said. "He'll be our starter Monday night. We believe he can do everything that a quarterback is asked to do."