Ashley Cole has been forced out of Chelsea's FA Cup replay with Manchester United in the 21st minute with an apparent hamstring injury. The severity of the injury is yet to be determined, but it was a bad enough pull (or tear) that Cole was unable to run it off. He was able to walk to the changing room under his own power.
The good news for Chelsea is that their replacement is probably the best backup left back in the league. Ryan Bertrand would start for most of the teams in the Premier League, and is good enough that he's been thrown into the side as a left midfielder just to get him into games on multiple occasions. The drop-off in quality shouldn't be too steep for Chelsea, but it's always tough for a defender to step into the middle of a game.
Some further good news for Chelsea is that Nani, the starting right winger for Manchester United, is having a bit of a shocker at the moment. If that continues, Bertrand will have an easier time settling in.