There is plenty of excitement over the 19-year-old's arrival at the Emirates this summer, having spent last season on loan at St. Etienne.
But Keown says Saliba can't be expected to solve Arsenal's defensive problems on his own.
Quizzed if he felt there was unwarranted pressure on the Frenchman, Keown told the PA news agency: "There definitely is.
"It is one of those where I would have been happier if Arsenal had paid more money and brought him in straight at the start of last season.
"There have long-been questions over the Arsenal defence and Saliba was expensive, but he is also young and has not played that many professional games.
"It can't be expected that he will walk straight into Arsenal, right into the team and be on it from the off."