I thought front players played decently vs. Sunderland. Its the central midfield that's a problem. Carrick and Fletcher were so passive and conservative it was painful too watch. You can't break sides with 4 people, you just can't. At least not when they had time to set up defensive perimeter, which Sunderland could comfortably do having a lead.
United are extremely easy to read when in possession. The only thing central midfield is capable of is playing sideways passes (and even that is quite adventurous by their standards). As a consequence Nani/Young or whoever is on the wing are always outnumbered. You can hardly blame Hernandez for not working the channels when nobody in entire side can play a through ball and the only thing coming his way are desperate unprepared crosses.
What else would you expect from Carrick/Fletcher/Cleverley? They've been taught to play football that way in the past 3/4 years. Ever since Scholes left, there hasn't been any creativity in midfield - which is why a Kroos/Carvalho or Fabregas/Carvalho partnership would work wonders.