Manchester United caretaker manager, Ole Gunnar Solskjaer, admitted that his side didn't play well against Reading on Saturday.
The Red Devils beat the Championship side 2-0 at Old Trafford but Reading were the better side.
Speaking to BBC Sport after the game, the Norwegian said: "There were nine changes," said Solskjaer. "It's never easy to get the rhythm and the tempo and the relations because they take a touch and then look at each other, they're not used to playing with together.
"Even though we train together it's different when you're out there at Old Trafford. So, that was down to me a bit and again I need to see them out there to see how they reaction when we concede a goal. We struggled today but we came through."
"It was a tough game, but 2-0 and a clean sheet [is good]," the 45-year-old said in a separate interview with MUTV.
"I made it tough for us because when you make nine changes it is never easy for a team that has never played together so delighted that we are in the next round.
"We work together every single day, every single game and of course we know we need some fitness work and work on relations but today I put a team together that has never played together and we are through so that is good."