McDermott's Premier League new boys were on course for at least a 2-2 draw in their opening away game at Stamford Bridge when the officials allowed an offside-looking Fernando Torres goal to stand. The Royals went on to lose 4-2 on Wednesday night.
Admitting he had made a beeline for the assistant who kept his flag down after the final whistle, a "gutted" McDermott said: "He (Torres) was offside and it was not a good night for him (the assistant).
"It's not been a good night for me, either."
He added: "I just said to him he'd got it wrong.
"My gut feeling was that it was offside, and it was clearly offside. Unfortunate for him.
"He said he'd have a look at it. He can have a look at it. That's life.
"People makes mistakes, and he's made a mistake tonight.
"It's just a shame it was for such a crucial goal because we would definitely have got a point.
"I'm really disappointed for the players.
"We didn't deserve to lose the game. I thought we were going to win it when we got to 70-odd minutes."
Indeed, Championship winners Reading held a shock 2-1 lead until the 69th minute, having bounced back from Frank Lampard's penalty through Pavel Pogrebnyak and Danny Guthrie's first goals for the club.
The latter came courtesy of a Petr Cech howler but it was not only the Chelsea goalkeeper and officials who messed up.
Reading keeper Adam Federici had a nightmare of his own for the second time in five days to gift Gary Cahill an equaliser.
McDermott said: "I'm not concerned about him at all. He's got fantastic mentality.
"It's like everything: once a result has happened, you have to box it off. You can't affect that. You have to move on.
"He's not overwhelmed by the Premier League. It was just one of those things.
"The shot's moved and it's gone in. That's life. Nothing we can do about that now."
Chelsea boss Roberto Di Matteo said he had yet to see Torres' goal, which also allowed Branislav Ivanovic to kill off Reading on the break.
"I looked at the linesman straight away and he gave it straight away, so I can't comment," said Di Matteo, who was pleased to see his side go top of the Premier League for the first time since November 2010.
He said: "We are very pleased with six points after two games.
"I was very happy with the team performance for long periods of the game.
"We lost a bit our composure after Reading scored the equaliser and second goal, but we'd started off very well, with some great football, and finished off very strongly."
The positives included another standout performance from new signing Eden Hazard, who helped himself to two more assists to add to his pair at Wigan on Sunday.
Di Matteo said: "Hazard had an impact in our team.
"He's finding his feet very quickly, so that is pleasing for us."
The negatives were all too apparent, with Chelsea's defence looking anything but solid and captain John Terry enduring a particularly tough time.
"At 2-1 down, we were very offensive and maybe lost our balance a bit, because we were behind," said Di Matteo, the latest manager to be charged with making the Blues more adventurous.
"The balance was always going to be the key.
"It's all great and everyone wants to see a lot of flair play but, to win games, you need a good balance to it. That's going to be the challenge."
Di Matteo was confident David Luiz would be fit for Saturday's game with Newcastle after the defender missed this match with a knee problem.
Luiz ruled himself out publicly on Wednesday morning on Twitter and Di Matteo revealed the club would deal with the potential breach of confidentiality "internally", likely meaning a warning.
Di Matteo refused to comment on reports Cesar Azpilicueta was on the brink of signing for Chelsea, while McDermott gave short shrift to suggestions Reading were trying to buy the right-back's Marseille team-mate, striker Jordan Ayew.
He said: "I heard that too as well. I don't think so..."