Watch the replay, watch his movement, his workrate. He missed a good header when he just came on too, well actually, the goalie saved it, but I thought he put himself about and did well. Borini drew a defender for Mario's goal too. Ok, perhaps excellent is a bit over the top, but my point is he did quite well. Also as LFC Chris has explained in his post above, Borini was the logical substitution to make, not Ricky.
I agree, Borini offered some mobility up front. I think we're struggling to adapt to playing with a relatively static Balotelli so to play both him and Lambert would have really limited our play.
