What is Achilles tendinitis? Explaining 49ers star Christian McCaffrey's injury and recovery timeline

Absent from the 49ers' Monday night matchup against the Jets in Week 1 was reigning NFL Offensive Player of the Year Christian McCaffrey. The running back was ruled out just before kickoff in a surprise move, much to the chagrin of fantasy football managers and San Francisco fans — and despite optimistic reports leading up to the game on his status.

In early August, what was originally labeled a calf strain kept McCaffrey from appearing in any preseason contests. After several weeks of rest and recovery leading up to Week 1 and even the running back himself saying he expected to play, head coach Kyle Shanahan appeared to act out of an abundance of caution when leaving McCaffrey out of the team's starting lineup against New York.

Shanahan revealed Tuesday that Achilles tendonitis has been the primary culprit keeping McCaffrey off the field recently, and that it "comes and goes in severity."

Here's everything to know about the setback, including explaining McCaffrey's injury and recovery timeline.

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What is Achilles tendinitis?

When the Achilles tendon, the tissue that connects the calf muscle to the heel bone, is overly stressed, that can result in Achilles tendinitis. While running, the Achilles tendon helps to facilitate calf muscles raising the runner up on their toes. Constant repetition of this motion combined with insufficient rest time can cause inflammation of the tissue, resulting in swelling, heel and ankle pain and stiffness or tenderness of the tendon.

According to the Cleveland Clinic , nearly 25% of athletes will develop an Achilles tendon injury in their lifetimes. Left untreated, Achilles tendinitis can cause Achilles tendinosis, which is when the tendon begins to degenerate, or an Achilles tendon rupture. The combination of rest, ice, compression and elevation should eventually minimize symptoms of tendinitis, and physical therapy can help as well.

When will Christian McCaffrey be back?

When Shanahan announced McCaffrey's tendinitis Tuesday, he said the running back would be day to day this week ahead of the 49ers matchup with the Vikings.

"The Achilles is tendinitis and that stuff comes and goes," Shanahan said . "And when it is acting up, it’s something you’ve got to be very careful about. Christian's very diligent about that stuff."

Shanahan also said that if the Week 1 matchup was a playoff game, McCaffrey would have suited up. But in order to keep his star player healthy, Shanahan elected to protect McCaffrey's health for the time being.

"If it was a playoff game, he made it very clear to me he believed he could go," the San Francisco head coach continued. "But when you hear that type of stuff and it's not a playoff game and it's week one and especially when you're dealing with the lower extremities like that, it was a tough decision, but hearing all the words in the long run, it made it easy.”

MORE: Jordan Mason, Kyle Shanahan disagree on timing of Christian McCaffrey news

49ers RB depth chart

With McCaffrey sidelined, second-string back Jordan Mason led San Francisco's running game against the Jets. The Georgia Tech product had the best game of his three-year career, racking up 147 yards on 28 carries for one touchdown in the 32-19 win.

Behind Mason are former Packers back Patrick Taylor Jr. and rookie Isaac Guerendo from Louisville. Here's a look at the 49ers' running back depth chart for 2024.