Boro opened the scoring just before half-time when Martin Braithwaite capitalised on Jordan Spence’s mistimed clearance, before Patrick Bamford doubled the hosts’ lead six minutes after the interval.
But McCarthy, whose side had recorded back-to-back victories before the trip to Teesside, believes that both goals could have been avoided.
"We should have done better"
“The overall assessment is I think we let them off the hook on 44 minutes, and I thought we let them off the hook again in the second-half after six minutes,” said the Town manager in his post-match press conference.
"I don't think Bartosz [Bialkowski] had a lot to do before they scored. They had a lot of possession in front of us, but we had a couple of half-chances we should have done better with.
“They were really, really poor goals. The first one we didn’t stop the cross, that gives them a real lift before half-time and the second goal was just bonkers, but having said that it was a £10 million striker who put the ball in the net.”
When asked if he felt his side had frustrated Boro in the first-half, McCarthy answered: "We frustrated them, there's no question about it.”
“We’d been able to nullify most of their threats and we may still have been able to do so had we not given a stupid goal away on 51 minutes.”
Shot-shy visitors manage just one effort on target
Despite the result, Town remain the second highest scorers in the Championship with 35 goals, yet they managed just one shot on target on a frustrating afternoon in the North East.
McCarthy was asked if his side’s subdued response after falling 2-0 behind concerned him, to which he replied: "If we give some of the teams beneath us those types of chances and those openings then we'd find it tough against the teams in the bottom half.
“But to give it to a team with Middlesbrough’s quality and the squad they have - I’m not going to say it’s impossible because had we got one and got it back to 2-1 it would've probably got nervous again, but we didn't.
"After giving them the second goal, they looked like a bloody good team and we looked like chumps chasing it around. That's unfair to us, but it's our own downfall and our own undoing."