Earlier this summer the Boro skipper had an operation to correct the issue, which he had been carrying for the last two months of the season, though further scans have shown that the injury is worse than first thought.
Boro captain played a key role last season
The tenacious midfielder played a pivotal role for Boro in their promotion winning season last campaign, and in the past two years he has formed a robust partnership with Adam Clayton in the middle of the park.
Yet despite his position as captain, Leadbitter, who joined Boro in 2012, would have faced undoubted competition due to arrival of Dutch midfielder Marten de Roon.
De Roon joined Boro for £12 million from Atalanta last week and has been part of a huge overhaul following the Teessider’s promotion to the Premier League.
Aitor Karanka will hope to give some of the new arrivals, which include goalkeeper Victor Valdes and former Ajax winger Viktor Fischer, a run out when his side travel to Doncaster for a pre-season friendly this Saturday.
Husband and Ayala also side-lined
Another man who won’t feature, however, is left back James Husband, after the 22-year-old dislocated his shoulder during 6-0 victory over York last weekend.
Husband is likely to miss the rest of pre-season and will require surgery to resolve the issue.
The defender has found it difficult to break into the first team due to the consistency of fellow full-back George Friend, and last season Husband spent time on loan at Fulham and Huddersfield in the Championship.
"That is the worst thing when you are playing in pre-season” said Karanka after the York game.
“For me, these kind of games are really just a training session, and when you go home with an injury, it is not good.
“Sometimes, these kind of injuries come from a stupid thing. I hope that Hubby is back with us as soon as possible."
Centre back Daniel Ayala will also miss the Doncaster game with an ankle injury.