Forster was injured during the Saints’ 2-0 home victory over Burnley on Saturday and a scan on Monday showed he had suffered damage to his left patellar tendon.
The 27-year-old has now had a successful operation and will miss Saints’ final eight matches of the Premier League campaign as a result.
An injury of this nature can take between nine and 12 months to heal, but Southampton will make a longer-term prediction of when Forster will return to action having assessed once the effects of his operation have settled down, with the club’s medical staff confident he will make a full recovery.
Southampton’s executive director of football Les Reed told the club’s official website: “Fraser is an outstanding goalkeeper and has been a huge part of our success this season. While this is a challenging situation, we look forward to him returning to action as soon as he is fully fit.
“I am sure he will go on to make a significant number of appearances for Southampton and for England, and will get the best support available to help him recover to do so.
“No one will be any sadder than Kelvin Davis, who did an outstanding job replacing Fraser in such circumstances. Nobody could be better to step into the fray in terms of experience and ability.
“Kelvin is a big-game player, who will be ably supported by Paulo Gazzaniga and Cody Cropper. I am sure the team will now be even more motivated to ensure we stay miserly in front of our goal and ruthless at the other end.”
Forster, who has been ever present for Saints this season after moving from Celtic for £10m last August, crumpled to the ground when his left knee appeared to give way as he cleared a ball upfield under a challenge from Burnley striker Sam Vokes.
He immediately signalled to the bench that he needed replacing and was carried off to a standing ovation from the home fans at St Mary’s before being taken to hospital.
Forster was forced to withdraw from the England squad for the upcoming games against Lithuania and Italy and he will also now miss the matches against the Republic of Ireland and Slovenia in June.