“We know the players are doing well, they are improving and of course big teams look to other teams to see what players they have.
“But we will do everything we can to keep the players in.”
Koeman has played down rumours of a rift with midfielder Dusan Tadic after claims the Serbian was seeking a move due to a lack of game time.
Koeman, whose side face Newcastle tomorrow, said: “I spoke to Dusan and he was totally surprised. He is one of the players that if he has a problem he speaks to the manager. There is no problem.”