The German goalkeeper is waiting to make his competitive debut for the Gunners after his summer arrival from Bayer Leverkusen.
"If you look at the statistics for goalkeepers in Europe, Leno excels in his distribution but is not the best when it comes to the shots he is expected to stop," Keown told the Daily Mail.
"The German was not in their World Cup squad and hasn't been called up for this international break.
"How good is he? We're not too sure yet."