Mitchell is dealing with a hamstring issue that may keep him sidelined for the entire preseason, Adam Schefter of ESPN reports.
Prior to kickoff of Friday's exhibition against the Packers, Mitchell wasn't seen in uniform along with a number of the 49ers' top skill-position talent, per Nick Wagoner of ESPN.com. But Mitchell's absence is related to an injury. While he may not make an appearance in a game in August, he is slated to be available when the team opens the regular season Sunday, Sept. 11 in Chicago.