It seems that halfway through his bout with Alberto del Rio, Rey Mysterio tweaked his knee. At one point, del Rio moved outside of the ring to taunt and essentially distract so that Rey could be properly talked to. He was walking around, putting pressure on it for a few moments, but continued the match regardless. Afterwards, when Punk and Cena were in the ring, Rey was still being looked at. It is believed that this is why Cena and Punk went on for as long as they did. He was taken away by officials, and Rey “was trying to keep any weight off his injured knee.”
Source: prowrestling.net
