The Saints boss admits they need their number one keeper back as soon as possible.
The goalkeeper has been out injured since March, having picked up a long-term knee injury towards the end of last season.
It was a disappointing end to what had been a fine first campaign in the Premier League for the goalkeeper following his move from Celtic, and he has been sorely missed in the side since.
Koeman, though, says he is set to return in around a month’s time, after beginning training sessions with coach Dave Watson.
“He’s doing well good, everything is going to plan, it is fast,” he told saintsfc.co.uk.
“He worked very hard and is already training with Dave Watson and that’s real goalkeeper sessions. He can be a part of the group in four, five weeks. He’s coming back from April, which is a long time.”
The Saints boss has admitted that the side are desperate to have their number one keeper back as soon as possible.
In Forster’s absence, on-loan Maarten Stekelenburg has managed to prove an able replacement, keeping four clean sheets in his 10 appearances so far this season.
He has been playing behind a defence that has largely performed well, though, with the likes of Virgil van Dijk in particularly fine form, and Koeman was full of praise for his squad, saying they need everybody to be successful.
“Fraser is a great goalkeeper,” he added.
“Everybody is happy how he is and how he is recovering well. We need him tomorrow, not in the future. We need everybody. We have a good squad, for the tough Premier League.”