Allardyce told Bolton's official website: "He's well versed with the demands of The Premiership and he can bring a lot to the squad, in terms of wide left, wide right or centre midfield.
"Once he gets an opportunity to play in the side I will expect him to show the sort of form he showed at Southampton, particularly under Gordon Strachan.
"If he can try and keep hold of the shirt and make life difficult for the person who he has replaced then we'll have a valuable player for the team this season."
But Fernandes nearly didn't make it to Bolton as Allardyce initially looked set to sign Olivier Bernard, only for the defender to go to Rangers instead.
Allardyce said: "I liked the pair of them but I could not sign both.
"I chose Oliver for defensive cover, which we were light on, but he then turned us down.
"But Fabrice also has experience in the Premiership and that could be crucial to us."
The Frenchman insists he is fit despite a lack of games and could play a part in Sunday's game against Blackburn.
He said: "I did a full pre-season and I took part in some friendly games.
"Now I am looking forward to starting over again with Bolton. Sam Allardyce has given me my confidence back.
"I have been in the Premiership for four years. If I can do something for this club I will be very pleased."
Fernandes has linked up with his former Southampton team-mate Kevin Davies, a striker he rates highly.
"Kevin is a good player and has made a big impact up front," said the Frenchman.
"He plays well in the air and scores goals. He is very important for Bolton as he also works hard for the team."