The hugely promising Spain Under-21 keeper was the man identified by Sir Alex Ferguson as the best option for the position after exhaustive scouting around the world.
With Edwin van der Sar retiring, the new arrival seems likely to be handed the no.1 shirt and Stepney feels he will soon pick up the language in Manchester.
"He's done very well for Atletico," Stepney told ManUtd.com. "His record last season [24 clean sheets in 45 games] is incredible in a top-class league. I've only seen him on TV and you can't tell everything about a player on TV.
"For example, I always like to see what a goalkeeper's doing position-wise when the ball's down the other end of the pitch and the cameras rarely show that. But certainly I've seen what a good shot-stopper he is and it would seem that he's going to be Sir Alex's no.1.
"We've also still got Anders Lindegaard, Ben Amos and Tomasz Kuszczak on the books, of course, and one of them will have to leave, either on loan or permanently. A club like Manchester United has to have three top-class goalkeepers because if your first-choice gets injured, you need quality cover in the team and on the bench."
1968 European Cup final star Stepney expects the keeper to quickly adapt to his new surroundings in plenty of time for the big kick-off in August.
"It doesn't take long for players to settle in at Carrington, even if they don't speak English," he explained. "United is so geared up to help them in and learn the language if necessary so all parties, the player and the club, can get the maximum