"Matthew's got a cruciate ligament injury again, which is a huge blow for him and us," Southgate told the Evening Gazette.
"It will require surgery and it will mean probably nine months or so out of the game.
"All we can do is support him in every way we can and find the best treatment for him.
"I think for a long-term injury like that dealing with it mentally is the toughest thing and it's very frustrating for him to have battled back from it twice before.
"His attitude to re-hab on the previous occasions has been brilliant."