If you're in the UK:
http://www.bbc.co.uk/iplayer/episode/b078xzgv/the-fa-cup-201516-everton-v-manchester-united
2:09 onwards. It started off with a poor/underhit through ball from Rooney (as always, remember that Swansea game earlier in the season?), Carrick diving into no man's land and then Herrera seeing the danger (from the edge of Everton's penalty box no less), chasing back and bringing him down very cleanly.
Barkley was in full stride and was 5-10 yards away from being able to isolate Blind and/or give a through ball to Lukaku. If you want to analyse the situation further, Herrera had a couple of choices 1) he maybe could have made a tackle as Barkley first came across him but he probably would have had to go in with his weaker foot first (left leg) and thus would have less leverage as it's more unnatural meaning he would have probably injured Barkley or 'worst', not connect with the ball and let Barkley advance forward. 2) He could have chased Barkley further until he gets leverage to make a tackle, in which case a) that's assuming he can catch up to a fully in stride Barkley and b) by that time the foul would be nearer to the penalty box.
You're absolutely fecking mad to criticise Herrera in this situation.
It's mind boggling how people watch/view football here.