Everton striker Dominic Calvert-Lewin will be out for up to two months after suffering an ankle injury on his first Premier League start.
Calvert-Lewin, 19, was named in the team for the victory over Southampton on Monday but had to come off after 12 minutes.
And speaking at a news conference ahead of his side's FA Cup tie against Leicester City at the weekend, Koeman said: "Calvert-Lewin is out for up to seven to eight weeks."
The manager also confirmed that James McCarthy had not yet resumed training following a hamstring injury, while goalkeeper Martin Stekelenburg also misses out.