Manchester United manager Ole Gunnar Solskjaer said his team's defending in the first half of Wednesday's 2-1 Champions League defeat by Istanbul Basaksehir was "unforgivable".
United went behind after 12 minutes in Turkey when Edin Visca's pass found Demba Ba in space at the halfway line and the striker ran forward before finishing past Dean Henderson.
Basaksehir doubled their lead before the break when Juan Mata lost possession and Ba turned provider for Visca to score.
"The first one is that we play a short corner and forget about the man up top. That's unforgivable ... We've not done our roles, it's my responsibility," Solskjaer told reporters.
"You don't see goals like that at this level. You shouldn't concede easy goals like that. When you do, it makes winning matches a lot harder."
Former United midfielder Paul Scholes said their defending was "embarrassing".
"It's completely down to organisation. It's like under-10s football. What the defence were doing I have no idea," Scholes told BT Sport.
"The goal is comical, laughable."
The defeat came on the heels of Sunday's 1-0 Premier League loss to Arsenal and while the results have prompted speculation Solskjaer's job could be on the line the Norwegian said it was not the time to panic.
"It's early on. Opinions are there all the time. We have to stay strong," Solskjaer added.
United travel to Everton in the league on Saturday.