Having fallen behind early on to a Diego Costa header, the visitors restored parity within eight minutes as Dusan Tadic tucked home a penalty following Nemanja Matic's clumsy tackle on Sadio Mane.
Mane and Shane Long were among Southampton's best players, but Koeman reserved special praise for Forster, who denied the likes of Eden Hazard, Willian and Loic Remy with impressive late saves to salvage a point.
"[Forster] was fantastic in the difficult moments," Koeman told the club's official website.
"He helped the team. We know he's one of the best in Europe."
Koeman named a positive attacking side in a bid to take advantage of any Chelsea fatigue following their UEFA Champions League last 16 exit to Paris Saint-Germain after extra-time on Wednesday.
While Southampton were forced to soak up plenty of pressure from the hosts after the break, the Dutchman was left feeling "very positive" at taking a point back to the south coast.
"It's very positive. A great result from a difficult match," he added.
"We had a fantastic reaction after the fast 1-0 that they scored. I think we were the dominant team in the first half.
"We had the best chances to score the second one after the 1-1 but in the second half it was more difficult.
"Shane Long and Sadio Mane played well and it was a good tandem in front."
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