How exactly he injured himself, the severity of the tear and even what knee it occurred in is unclear. Yet the situation necessitated Federer going under the knife, as he announced on his Facebook page:
Tennis (@ESPNTennis) February 3, 2016
As he recovers and begins rehab, Federer will skip two tournaments this month: Dubai, where he's a seven-time champion, and Rotterdam. By sitting out Dubai, 500 points will fall off his ranking total, though he'll still own a comfortable lead over world No. 4 Stan Wawrinka.
Not that Federer frets too much about his ranking these days.
The bigger concern is how much time the Swiss star will miss. While he's slated to play next at Indian Wells in March, that's no longer a guarantee. Federer had planned a two-plus-month break after that tournament to rest ahead of the French Open in late May. Now his calendar is very much in flux.
Federer could enter an extra clay-court event or two (and possibly Miami in late March), but how sharp will he be? Besides presenting the physical challenge of getting his knee right again, the injury also interrupts his training schedule. For a player who likes to peak as summer begins, an extended layoff could put Federer behind the curve.
In 2013, he suffered a lingering back injury that slowed his training and play for several months, as he recounted to Kamakshi Tandon of Tennis.com: "Just maybe feeling that training was lacking from the weeks I lost in April. Just I could feel for a while I was playing the wrong way. Little things crept into my game that shouldn't have and probably wouldn't have had [I] felt better."
Will he encounter a similar level of rust and lack of precision this time?
He can ill-afford to, especially with this season taking on so much importance.
Beyond his goal of winning Wimbledon for an eighth time and snapping his Grand Slam slump, Federer is eager to participate in what should be his final Olympics run. Aiming to take home singles gold for the first time, Federer will also play doubles and mixed doubles in Rio de Janeiro. That could change if he's not at 100 percent by then or opts to reduce his workload.
Of greater consequence is how this situation impacts Federer's long-term future. At 34, he only has a handful of seasons left before walking off into the sunset. If he struggles to regain strength in his knee or can't move as freely, his ability to remain a top contender will be severely hindered.
As it is, he's already struggling to keep pace with Djokovic (like every other player). Any sort of reduced effectiveness could spur him into retiring sooner.
Not to be too rash—Federer could return in a few weeks without missing a beat—but we've never really seen him deal with an injury of this magnitude before. His prior troubles, like the one in 2013, were limited to his back. Knee problems (just ask Rafael Nadal) can become chronic and affect athletes differently. When surgery is required, it adds an extra layer of uncertainty.
If anyone can bounce back and return from injury, it's a tireless worker like Federer. But there's no denying this obstacle will be a delicate one to clear.
All statistics are courtesy of ATPWorldTour.com unless otherwise noted.
Joe Kennard is a featured columnist for Bleacher Report.