Haller has been excellent for the Eredivisie side since joining midway through last season.
Mane has been attracting interest from Manchester United and Chelsea in recent times, but Koeman initially wanted to hold onto the Senegal international for a while longer (via Star).
However, the Dutchman is now resigned to losing the winger much sooner, the Star reports, after he turned up late for a meeting ahead of the Premier League clash with Norwich. The Saints would go on to lose the game 1-0 to register their sixth defeat in their last eight league games.
The prospect of losing one of their best players is something that fans of the south coast side have become accustomed to in recent years. But, as when the side lost the likes of Luke Shaw, Adam Lallana and Morgan Schneiderlin, Mane's replacement has already been lined up, with Koeman once again looking to someone who is shining in the Netherlands.
Having recruited the likes of Graziano Pelle, Jordy Clasie and Dusan Tadic from the Eredivisie in the past two seasons, it is hardly a surprise that Koeman has looked to his home country for talent.
Haller has been exceptional for Utrecht this season - having made his loan move permanent at the end of the last campaign - scoring 11 goals in 17 games to help the side move into contention for a spot in the Europa League next year.
The 21-year-old is already developing into a special talent who is particularly strong in the air. And Koeman will be hoping that, should he be able to complete a move for the youngster, he can replicate the blistering form he has shown for his current side.