Lita sustained the injury in the draw at Portsmouth on Saturday.
“It’s disappointing because Leroy had been playing so well, but we know from plenty of experience that injuries happen and we have to get on with it,” Middlesbrough manager Tony Mowbray said.
“Leroy has never had a hamstring injury before so he was unable to tell us how bad it was on the way home on Saturday.
“But from my experience, hamstring injuries are usually four to six weeks.
“Obviously you never want to be without your goalscorers, especially when they are in form and you are fighting a battle like we are.
“But it’s an opportunity for somebody else to come in and we will keep on pushing on.”