Villas-Boas rebuffed a bid from Blackburn Rovers to re-sign veteran goalkeeper Friedel as a player-coach.
"Brad has been excellent, I spoke to him and he has been such an important player for us," Villas-Boas told the club's website. "Rotation has not been because of performances.
"Both goalkeepers' performances have been excellent. It gives me the ability at any time to throw a different keeper into the game.
"I don't know anything about (Blackburn's bid). We are trying to hold onto our best assets and Brad is certainly one of them."