Ole Gunnar Solskjaer’s side fell to a 2-1 defeat against Wolves in the Premier League on Tuesday to fall to sixth place in the table.
The Red Devils have now lost three of their last four games in all competitions and their next task is a Champions League quarter-final first-leg clash with Spanish giants Barça at Old Trafford next Wednesday.
“Manchester United have hit a bad patch of form at just the wrong time and it's been Wolves who have inflicted damaging defeats on them twice," former United striker Berbatov told Betfair.
“With all due respect to Wolves, while they're a good Premier League side, they aren't of the calibre of City or Spurs. Certainly not of United's next opponents, Barcelona, and of course we need to ask questions about my old side right now.
“I'm concerned by United's poor form. 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. 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 Barça.
“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. Sir Alex Ferguson sometimes didn't need to motivate us for the big games, his job and real skill was to motivate us for the small games.
“I admit, Barcelona are the rightful favourites in the tie, but of course so were PSG, and nothing is certain in football. United can forget their recent form and make a statement here, but - and it's nothing new or original - they must watch out for Messi.”
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