"I do think Berbatov has to start justifying his £31million price tag if United are to get back on top and win a third successive title," Robson told the Mail On Sunday.
"Berbatov played too deep at Emirates Stadium and therefore wasn't a goal threat at all.
"He never looked like scoring and his record of one league goal in nine games for United suggests that has been a problem since he arrived from Spurs.
"He doesn't look like the same player to me that thrilled us at Tottenham.
"Berbatov has shown some sublime touches at times, including that fantastic turn to set up a goal for Cristiano Ronaldo against West Ham.
"But a United number nine has to provide goals as well, particularly as Sir Alex has had to leave out Carlos Tevez to accommodate him.
"I watched Berbatov drop back virtually to the halfway line at times yesterday, content to get a few touches of the ball and play some simple passes.
"I bet Arsenal were delighted to see him so deep because he wasn't going to hurt them from there."