The Bulgaria international has scored just 26 goals in two seasons and his performances have been widely criticised since his £30.75million move from White Hart Lane in August 2008.
"I don't want to knock him. He hasn't come up to the standard he set at Spurs, but when he was there he looked an extremely good player," said the Scot, who scored 237 goals in 404 United appearances.
"He's always had that laid-back style, even when he was at Tottenham, and that's what the fans admired him for but now he gets criticised for it."
Law said he hoped Berbatov did not turn out to be another expensive misfit like £28million midfielder Juan Sebastian Veron, who joined from Lazio in 1999 but lasted just two years at Old Trafford.
"He looked an excellent player with Argentina and in Italy, but when he came to United he didn't do it," Law told the Manchester Evening News.
"He was a commanding player, yet when he came to United he wasn't."