The 27-year-old has returned from a near-10-month absence to keep four clean sheets in a row for the Saints with his best individual performance coming in Tuesday night's 0-0 draw with Arsenal.
Forster made a host of top quality saves at the Emirates Stadium, in particular denying two close range efforts from Mesut Ozil, which prompted high praise from Koeman who feels he could be one of the top glovemen in England.
"You need a goalkeeper who is magic and Fraser was magic tonight," he said on Sky Sports.
"It's always difficult because he was nine months out and maybe we had to force it a little bit because [Maarten] Stekelenburg was injured.
"If he was not injured, maybe we would wait one or two weeks more, but sometimes it happens. We know, normally, if you are nine months out you need minutes, but from the start he has been what he was.
"That's unbelievable and it's a credit to the medical staff and Fraser himself. He did everything and he worked so hard to come back.
"He is one of the best goalkeepers in the Premier League and maybe one of the best in Europe."
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