Well, bigger attacking threat than Schneiderlin and Carrick isn't that much of a feat as they both are meant to be United's more defensive midfielders. Especially Carrick. His main role when United has the ball is to provide a passing option. I'd like to see him take a shot at goal more often, though - he's not half bad at it.
But I agree, a Di Maria replacement is needed. Especially as Valencia currently isn't half the winger he was a few years back. I don't like to see Mata out there, he belongs in the middle.