"Gary Megson has had criticism, which beggars belief," Fergie said ahead of United's match against The Trotters at Old Trafford on Saturday.
"Bolton are a better team than people give them credit for. They have always been a determined side.
"But when Sam Allardyce was there he elevated them to a higher level. When he left, he was a big man to replace.
"But Megson has kept them up. They are physical and determined. They never give in."