The playmaker has not featured since playing the first 69 minutes of the 0-0 draw with Arsenal in early February.
“In my opinion, every player in the squad is an important player,” Koeman told reporters at his press conference ahead of the visit to AFC Bournemouth. “Maybe it’s less attention now because Dušan is not playing from the start in the last few games, but it’s all about competition.
“Also for him it makes it more difficult when we changed the system. Of course if you play a 3-5-2 or you play a 4-3-3 that means we have one less position in front.
“You play with two strikers and not with three strikers and of course that’s difficult for players like Dušan.”
Koeman added: “There is more reasons that he lost a little bit his position to start. One was about the system and the second was that, in my opinion, he wasn’t on his level what he can bring and what he needs to bring.
“He is fighting hard to come back and get that opportunity, and that’s what you like to have. Of course he will be disappointed. Every player who doesn’t start is disappointed. I think that’s a good way for a football player, but show me and show the rest that you are ready to play.”
Koeman also reiterated that Jay Rodriguez, who has returned to training, needs more time before being ready to play, and is likely to feature for the under-21s in what would be the next stage of his recovery from foot surgery.
“We have to look to the schedule of the under-21s and maybe he gets his first minutes for the under-21s, but it’s all about how he is doing this week,” said Koeman.