Arsenal’s Timber has been sidelined since mid-March with a groin injury and Koeman said his chances of making the Dutch squad were in the balance.
"Jurrien has been dealing with this for quite some time. It has been a rollercoaster ride,” Koeman told Dutch television on Monday.
"He is training again to see if he can make the Champions League final and be available for the World Cup, but it does not look rosy at the moment."
Koeman expects Depay to return to action in the coming weeks before joining the Dutch national team.
The 32-year-old striker, who is the Netherlands' record scorer with 55 goals, suffered a thigh injury at the end of March and has not played for his struggling Brazilian club Corinthians since.
"He still has three matches with Corinthians before we get together as the Dutch national team.
"He really needs to get minutes in those matches. I assume that will happen, and if everything goes well, he might even play 90 minutes in the final match. After that, we still have two weeks until our first World Cup match against Japan."
The Dutch already have to do without three injured players for the World Cup, where they also face Sweden and Tunisia in Group F. Defender Matthijs de Ligt, midfielder Jerdy Schouten, and attacker Xavi Simons all miss out through injury.