The John Wick star, 59, suffered a fractured kneecap after falling over while filming his new comedy Good Fortune and he was later pictured walking on crutches with an ice pack strapped to his leg.
Keanu has now revealed the injury happened when he took a tumble following an ice bath.
During an appearance on The Late Show with Stephen Colbert, Keanu explained: "I was filming a scene with Aziz Ansari and Seth Rogen and we were in a cold plunge.
"I was loving it, I was standing there, and we finish the scene, and you know when you're cold and you're (shuffling)? I had a bathing suit and a towel, and you put it over your head and you do the cold shuffle?
"I'm doing the cold shuffle in this room that had protective carpets down and then, just here, there was like a little pocket, and my foot got caught in the pocket in the shuffle, and then I went [down], but (my knee) didn't follow.
"And then, in slow motion, I went falling. My arms came out, but then my knee failed because it's got some stuff, and I spiked it. And my patella - kneecap - cracked like a potato chip."
Initially, Keanu was convinced he was fine following the fall but he later realised something was wrong because his knee was "blowing up".
"Comedy's hard, man," he quipped.
Keanu continued working on the film but his co-star and director Aziz Ansari later revealed they had to push back a scene involving salsa dancing until the actor's injury had healed.
During the interview with Stephen Colbert, the host asked Keanu if the knee injury was the worst thing that has ever happened to him and the actor replied: "No, the two-level fusion of my spine was."
The Speed star previously underwent spinal surgery shortly before filming his 1999 movie The Matrix after doctors discovered two discs in his back needed urgent attention because he was starting to lose his balance.
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