Well, tecnically he's not a replacement as Rooney has been named in the squad. Rooney is unlikely to recover in time though! In which case, Defoe or Bent may get called upon, maybe even Fowler if Sven's been on the happy pills again.
From what i've seen of Walcott he looks competent enough, although those were Championship games for Southampton. However, at this level defenders have no answer to pace! We all know what happened at France 98, when boy wonder Owen took the world by storm.
I just hope Rooney doesn't lie like Beckham and declare himself fit, when he isn't.