IIRC didn't McAteer declare himself fit in 2002 before the Cameroon game when he clearly wasn't. I remember him saying something some time after which went along the lines of there was no way he was going to rule himself out of such an important game.
With no disrespect to McClean I think this may be a similar situation with McClean unwilling to admit he wasn't 100% fit (no doubt fueled by a strong desire to take an opportunity to start) and Trap making the decision based on what he saw.
I was also a bit miffed why a lot of media kept on going on about McClean being left out purely because Trap was a bit mad and not mentioning anything at all WRT his previously reported illness.