Ferguson believes that 'Big Sam' was unlucky to have previously got the sack at Newcastle and Blackburn.
The Scot also believes his Hammers counterpart is unfairly judged for his sides' typically direct playing style.
Sir Alex Ferguson: "Big Sam went to Upton Park a year and a bit ago and the cynics said he's not a West Ham manager as he doesn't play football.
"What was his mandate?
"To get them up into the Premier League as quickly as possible and he got them up within a year. Tell me someone else who could've done that?
"He's also had to rebuild the team and get two or three players out because of their salaries.
"He's very underrated, it's as simple as that. There's a sort of impression about him being a certain type of manager. It's not really fair and I'm pleased he's back in the Premier League.
"The Blackburn situation was one of the craziest things I've ever seen but I think Sam was more disappointed with what happened at Newcastle. He was unlucky that a new owner came in and wanted a certain type.
"He didn't get a chance in his first really big job with the expectation and fan base up there - he didn't get a chance to do the job right."