Luke Shaw accepted that Manchester United were "awful" in the first half against AC Milan and even suggested they were lucky to go through.
United were indebted to Paul Pogba's influential display as a second-half substitute at the San Siro as they clinched a 1-0 victory — 2-1 on aggregate — to progress to the Europa League quarter-finals.
Ole Gunnar Solskjaer's side had been poor in the first leg at Old Trafford and again in the first 45 minutes. But they clicked into gear after Pogba's half-time introduction and his 48th-minute goal, which sealed the tie.
Shaw, however, believes United have many areas to improve in, despite the win.
Speaking to BT Sport after the game, he said: "Obviously very happy that we're through. I'll be honest, and I think the staff would agree, we weren't good at all. Especially first half, we were awful.
"I think first 10, 15 minutes we started well. We kept control of the ball but it was like a switch, we turned off and we were very sloppy, giving away balls and put pressure on ourselves. I think maybe we were lucky.
"But second half, a bit better. It always helps when you've got a world class player like Paul to come on.
"He made the difference. It's brilliant to have him back and, like I said, the most important thing was the result and getting through to the next round."
Shaw then called for United to build on the performance as they progress to the Europa League last eight and face Leicester in the FA Cup quarter-finals on Sunday.
"We need to be better than tonight," he said. "AC Milan are obviously a top side and we knew it would be hard coming here. The result wasn't in our favour.
"But it's a very good win and the biggest positive is that we're through and we've got players coming back and the squad is looking better.
"It's good times now and we've got another important game at the weekend."
