According to the commentators it was a tactical switch, Suso was even trying to line up that free kick before he came off, very bizare subtition as Suso was doing well up to that point. I'd love for Hendo to prove me wrong and grab a winner or something but meh I doubt it.
Good move, he wanted Gerrard up supporting Suarez more, arriving in the box, suso was getting plenty of chances doing exactly that, Gerrard would have finished prob one of them at least.