Harlequins director of rugby Conor O'Shea blamed his side's accuracy after their surprise 18-13 loss to London Irish.
The 2012 champions enjoyed the majority of possession in the opening period but were level at 6-6 with the Exiles by half-time.
Two quick-fire tries from new signings Blair Cowan and Alex Lewington then gave the Exiles an insurmountable lead after the break, as Quins picked up their third loss of the campaign.
"In the first half we had nearly all the territory and possession but we had inaccuracies all over the place," said O'Shea.
"I'm really frustrated by our performance as you can't go out and play with such lack of precision and control."
England scrum-half Danny Care grabbed the visitors' only try after coming on as a replacement for Karl Dickson, with O'Shea believing that the decision to start Dickson was correct. O'Shea also explained the early withdrawal of captain Chris Robshaw.
"Care is an individual genius but Dickson and he are both England squad members and there is little to choose between them," added O'Shea.
"Karl showed a lot of control and the responsibility for the defeat lay with the outside backs who had enough space to go round the opposition."
"Chris informed me at half-time that he had a knock on the head. He wanted to give more but I could see he was struggling so I replaced him."
Exiles boss Brian Smith defended his side's own inaccuracies in the win, along with the decision by fly-half Shane Geraghty to run a penalty rather than take a simple three points before the break.
"Shane is all ambition, it's very easy from the stand but at half-time I said that we needed to play in their half and take the points as we had a couple of chances to kick penalties in the first half but chose not to do so," said Smith.
"It was by no means a perfect performance but we found a way to get over the line.
"Every time we had a hurdle we found a way to get over it and I am very proud of the squad.
"It was a really courageous performance, especially after we lost Kieran Low after three minutes. We knew that they would throw the kitchen sink at us but you don't beat Quins by surprise or chance so we deserve a lot of credit."