49ers' McCaffrey has Achilles tendinitis in both legs

San Francisco 49ers running back Christian McCaffrey is dealing with Achilles tendinitis in both of his legs, head coach Kyle Shanahan said Monday, according to Adam Schefter of ESPN.

"I think it started in one (leg), you overcompensate, it went to the other. But mainly one of them has just been bothering him, but he's had it in both," head coach Kyle Shanahan said Monday, per Schefter.

Because of the bilateral Achilles tendinitis, his timeline to return to the lineup remains unclear, according to Schefter.

McCaffrey was placed on injured reserve on Sept. 14 after reporting the issue in his right leg, so the earliest he can return is for the 49ers' Week 6 contest against the Seattle Seahawks. It is unclear when McCaffrey will return to the 49ers lineup.

The 28-year-old didn't suit up for the team's victory over the New England Patriots on Sunday. McCaffrey flew to Germany last week to get treatment for his Achilles injuries, which he's dealt with since the summer.

"We'll probably turn it up here in the next couple of weeks. ... We'll start testing it out here sooner rather than later," Shanahan said of McCaffrey's injuries on KNBR radio last week, per David Lombardi of The San Francisco Standard.

George Kittle and Deebo Samuel returned to the Niners' lineup in Week 4 after missing their Week 3 loss to the Los Angeles Rams. San Francisco's offense ranks second in yards per game and seventh in points per contest to start the season.

Jordan Mason, McCaffrey's replacement, sits second in the league in rushing yards (447) and boasts three touchdowns.

San Francisco will face off against the Arizona Cardinals in Week 5.