The Atletico Madrid goalkeeper saw action for Spain in last night's 1-1 draw with England U21.
Keown told the Daily Mail: "United will like the fact he wants to pass the ball out from the back.
"And he did little wrong. He clearly has the necessary skills but until he fills out, he will not have the same physical presence as Edwin van der Sar. For that reason, signing him could be deemed a slight risk."