For me, it's the drop in mental concentration and focus that leads me to believe he is fatigued. We saw it against City and we saw it yesterday.
What i've noticed is that he tends to 'force' passes through a lot of traffic. Now typically it works, however at times he does it when he shouldn't, and frequently, leading me to believe he's mentally drained. When he does it, it leads to a counter attack from the opposition.
I'd drop him against Villa and save him for Everton.