Forster moved to Southampton from Scottish Premiership champions Celtic in the summer of 2014.
The England international goalkeeper cost £10 million in transfer fees, as reported by BBC Sport, and he lived up to expectations in his debut Premier League campaign.
The former Newcastle United prospect was a key figure in the Saints’ brilliant 2014-15 season, leading Koeman to speak highly of him.
“I think the goalkeeper, Fraser Forster, was a very important signing for the club,” the Dutchman told Southampton’s official website. “A really good goalkeeper, an international goalkeeper.
“He’s not the oldest which means we have a long time that we can have Fraser as our goalkeeper. That gave the team confidence.
“He played a big part in all the clean sheets in the first part of the season. That was an important one."
Forster made 30 appearances in the Premier League for Southampton in the recently concluded 2014-15 campaign, and would have played more had he not suffered a serious knee injury in March.
The Englishman kept 13 clean sheets in those 30 league matches, making two saves per game on average and 2.86 saves per goal on average in the process.
Forster probably did not get the recognition that he deserved, but there is no doubt that he will slot straight back into the Saints’ starting line-up when he eventually recovers.