United have suffered three defeats in their last four games as they prepare for their quarter-final clash with the LaLiga champions this week.
And United cult-hero Berbatov feels matters could get a whole lot worse for Ole Gunnar Solskjaer and his men based on their recent blip in form.
"I'm concerned by United's poor form and, if they continue to make mistakes like they have been doing recently, then, let's face it, they're going to be punished by Barcelona," Berbatov told Betfair.
"They are making their lives difficult at the moment and it's looking tough to make the top four in the Premier League now. United went on that incredible undefeated run when Ole Gunnar Solskjaer took over and this is their first bad patch.
"You know, when you're winning everything feels natural and good but an important thing is knowing how to react to a defeat. I'm sure Ole knows what to do here and he will have his side fired up and ready for Barca."
He added; "In football sometimes you have that strange thing where a club can falter against small teams but turn it on against big teams. It's crucial to get motivated for every game, not just the big ones.
"United can forget their recent form and make a statement here, but - and it's nothing new or original - they must watch out for that man Messi."