The United boss said on Friday: “His stats don’t show that [he doesn't work hard]. The stats are good.
“Like a lot of talented players, he’s economic with his running. His movement and ability to find space is fantastic; he pops up where defenders don’t want him to pop up.
“That’s what I always thought when he played against us. I used to think ‘Why has nobody picked him up?’ He’s got great awareness and finds good space in the box."
The Bulgarian has netted five goals for the Reds so far and set up several others, notably Cristiano Ronaldo's second against West Ham.
Sir Alex was full of praise for Berbatov’s contribution on Wednesday, but believes the striker still has more to offer his side.
"His awareness and passing ability in the last third of the field is absolutely outstanding, but I still don’t think we’re using him enough, I really don’t," insisted the Scot.
Video: Watch Sir Alex's press conference now on MUTV Online.