pereyra is not an effective player when it comes to any kind of defensive task. he can run, his position is fine, he's got tactical discipline, sure, but he can't tackle or steal balls too effectively, and easy to dribble past. hernanes is an other story: he used to play as a dm, and even at

he had some decent defensive stats for an offensive player. he could be more useful when losing possession than pereyra.
offensively, pereyra is faster, can dribble past opponents more frequently, and can create almost as good as hernanes. my main concern with our new guy is that he's never been a fast player, and he rarely hit his top form during the last 3 seasons, which apparently shows his age. he's a good passer, shoots fine with either feet (e.g. [video=youtube;r0LhXRbZi08]http://www.youtube.com/watch?v=wg2RVwYf27k"]left[/URL], [URL="http://www.youtube.com/watch?v=r0LhXRbZi08[/video]), he has a massive defensive workrate for an attacking player, so on paper, he could be more useful than pereyra. he has to hit top form for that, though.
just my 2 cents, of course