Hyypia said: "I think we will have to be a bit more clinical against Manchester United than we were on Thursday.
"I thought we were a little bit sloppy in the second half against Vitesse and we're not happy about that. Some of the chances didn't go in on the night but I hope we were saving the goals until Sunday.
"It wasn't difficult to erase the thought of playing United before the Vitesse game because we are professional, although I have to admit there were moments when I was on the pitch I remembered what was going to happen on Sunday.
"I told the Vitesse players they should be careful with us because we have a very important game coming up.
"I told the Vitesse defender Stefanovic when he kicked Michael Owen at the end of the game to watch it because we need everybody fit on Sunday.
"Everyone realises this is a very big game for us and now we've had the kind of result we needed."