Rio Ferdinand on the changes made for Tuesday's game with reading.
"A lot of the lads had not played for a while, Gaby (Heinze) has not played for almost a month and there are a lot of lads who have not been playing regular football and that shows the depth we have got in the squad at the moment," said Ferdinand.
"To come here, which is a difficult place, a bad pitch, is hard, but we started great and all in all we are through and we are happy."
United meet Middlesbrough in the last eight and Ferdinand wants to play at Wembley.
"Wembley is a new stadium this year, I have played at the old one and it would be great to be one of the first players to play there in the new one," he added.
"The aim is to win as many games as we can towards the end of the season and if we do that then we will be there or thereabouts in all the competitions."
O'Pie was also pleased and relieved by the win
"I know we let Reading back into it but we have to try to take the positives out of the win and some of our football at the start was sensational," said the Republic of Ireland international.
"To score three goals so quickly wasn't even in our wildest dreams. Reading probably wanted to get stuck into us during the early stages but we were buzzing and in less than six minutes they were three goals behind."
"We know we should have killed the game off,"
"Once their second went in, we knew we would have to defend for our lives because they were going to start pumping long balls into the box.
"It wasn't a question of being complacent because we knew they would try and make a game of it and they might have taken it to extra-time. The one that hit the bar at the end was very unlucky.
"Overall though, I think we deserved the result in the end."