United are likely to play a shadow team with their Europa League semifinal against Athletic Bilbao due on Thursday.
"They don't change their structure a lot (since that game). They are trying to find the best spots for every player," said Iraola on Friday.
"For sure, he (Ruben Amorim, United manager) is more familiarised than when we faced them there. I think when we played them the game was quite level, it was not a 3-0 game.
"I expect also a tight game. I hope we can win it, we can make the difference, but it is a team that normally defends quite well, doesn't give you a lot of chances.
"I know they have the game on Thursday in the Europa League but they have four days to rest, so we have been preparing against the best Man United team."
"I don't know what United will do"
Iraola added: "I don't know what they are going to do.
"I suppose for them Thursday is very, very important, but I am sure they want to beat us on Sunday also.
"And also Thursday is not like the second leg, it is the first leg after they will have to wait another week, play the decisive game one week later.
"We don't know obviously the players that are going to be against us, but we have prepared like they are going to put their strongest 11, whatever he considers."