Carroll, 36, who was at United from 2001 to 2005, watched his new side beat his old team 2-0 in Athens last month.
He told BBC Radio 5 live: "It was a big shock, especially the performance of them in the first leg.
"I have never seen a Manchester United not create many chances.
"They have lost a lot of games and let a lot of goals in.
"We have great respect for them. We know what they can do. If we can keep them quiet hopefully the fans will get on the back of the players and make them more nervous.
"Manchester United can turn it around. It is very important for our players to go out and try to get a goal."