Reading took the lead in the first-half through Jimmy Kebe's header but Preston were much improved after the break and drew level when an Eddie Johnson header hit the post and went in off the back of McCarthy.
The England Under-21 international has been outstanding since stepping in for Adam Federici and had his first bad moment in a Royals shirt.
McDermott said: "Alex was ill this morning and hasn't had any sleep and he was very unlucky with the goal.
"So he's had a bad day but he's done very well for us."
Source: PA
Source: PA