The Royals' boss was disappointed with the assistant referee after he failed to spot the Spanish striker was "clearly offside" for the Blues' third goal in a 4-2 win
The Royals surrendered a 2-1 lead after Pavel Pogrebnyak and Danny Guthrie put the visitors in the lead following Frank Lampard's opener from the penalty spot.
But Reading couldn't hold onto their lead as goals from Gary Cahill, Fernando Torres and a late Branislav Ivanovic strike sealed the win, but McDermott was fuming that the assistant referee did not see Torres in an offside position, as he gave them the lead.
"We didn't deserve to lose and I thought we were going to win it," McDemott told reporters.
"I'm gutted the linesman has got the decision wrong for the third goal. It's not a good night for him, and unfortunately that means it's not a good night for us.
"It was clearly offside, which is really disappointing. People make mistakes and he made one tonight."