The Gunners paid £27m to St. Etienne for Saliba's signature last summer and then immediately loaned him back to the Ligue 1 club.
There is a lot of hype around him moving to London for next season, but Keown doesn't understand why he didn't play at the Emirates last season.
"There's an awful lot of work to be done in there [defence]," he told talkSPORT. "There was the defender from Saint-Etienne William Saliba who they paid £27million for but was loaned back again.
"I couldn't quite work that out because now I'm being told he's world-class.
"Never mind if he's world-class, if he was Premier League class it would have been good enough because that was an area of concern."