Barca go into the game on the back of consecutive defeats to Real Madrid.
"We obviously need to give a little bit more. We have been losing recently but we know what we have to do to change things," Messi told the club's official website.
"Losing hurts. We'll try to get over this situation. There's a very important match (against Milan) on the way.
"We know what the problems were in the games we lost. We talk about these things in the dressing room, and that's where they'll stay."