United confirmed on Thursday that they had completed the capture of free agent Valdes, who has been training with the club and will serve as number two to De Gea.
The latter has displayed fine form under Louis van Gaal this season - with reports suggesting Real Madrid may be keen on signing the former Atletico Madrid man.
However, Valdes feels his arrival at Old Trafford will help De Gea maintain form rather than increase speculation around his future.
"I think now, in the moment, [De Gea] is the best goalkeeper in the world," the ex-Barcelona keeper told United's official website.
"His performance every day and in every game shows everybody that his level is very high.
"He has had a great year and it is very nice. Now I think he is the best goalkeeper in the world.
"I am here to help everybody. Always in my career, I have played to help my team-mates, to help my coach and to help everybody.
"I am a part of the team and I am not a problem, I am just a part of the team. If I play a game, I am always thinking about playing well and to help the other team-mates, to help them to win the match."
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