The midfielder has been a long-term target for Arsenal, however this week Chelsea have also joined the race to sign him at the end of the season.
The 25-year-old has now made 27 appearances for the Saints this term, and it is reported that the club will only be listening to offers in excess of £25m.
Koeman said: “It will be hard when there is real interest in the player.
“It is up to the player but I think Morgan is happy at the moment.”
The France international would greatly strengthen the midfield options of either Arsenal or Chelsea if they were to sign him.
Arsenal are currently second in the Barclays Premier League having accumulated 63 points, one point ahead of Manchester United in third.