The Dutch duo sustained the knocks in Ajax’s 6-4 defeat to Utrecht on Sunday. De Jong picked up a knock only minutes after the first whistle and was replaced after only three minutes of play, while Janssen managed to play through the pain barrier until the end, but complained about pain after the game.
The club's official website has now revealed that De Jong will be out for four to six weeks with a thigh strain and Janssen is expected to be sidelined for 14 days with a hamstring injury.
The pair joins Nicolai Boilesen, Kolbeinn Sigthorsson and Bruno Silva on the Ajax injury list.
The Eredivisie champions are currently fifth in the league, 11 points behind leaders AZ. They are second in Group D in the Champions League and face Lyon at the Stade Gerland on November 22.