The 21-year-old striker moved to Rangers from Tamworth in 2008 and made five first-team appearances last season.
Hemmings has had two previous knee operations and his absence will put a further strain on manager Ally McCoist's already thread-bare squad.
McCoist is thought to be a big fan of the striker and was planning on handing him a big role next season as the Ibrox club begin life in the Third Division.
With most of his top stars leaving this summer, and further cuts to the playing budget likely to force the departure of even more, Rangers will be going with a youthful squad next term.
And that would have meant Hemmings would have more than likely been leading the line next weekend when Rangers start their competitive season off against Brechin City in the Ramsdens Cup at Glebe Park.
Hemmings, however, still believes in his ability to become a first-team regular when he returns to action, possibly around the busy Christmas period.
"It's really frustrating to have had to go through another operation and it's annoying that I'm going to be out for so long again," he said.
"I felt good going into pre-season but then the problem flared up in the first couple of days and the surgeon said I needed surgery again.
"It's up to me now to make sure I work hard in my rehabilitation and get back to the level of fitness I would like to be at.
"I still feel I can get there, even after all the problems I've experienced, and I'm grateful to the club for giving me such welcome support throughout my injury problems."
McCoist will now have to have a closer look at his attacking options with every single recognised striker now unavailable.