Regular Wales captain Rees, 30, is out of action with a neck problem that is likely to require surgery.
“He has had an injection, but he is still in a lot of pain," Gatland said. “It looks like he will need an operation sooner, rather than later. We will have to sit down and discuss things with him.”
Former British and Irish Lion Rees captained Wales through last season’s Six Nations tournament and the Autumn internationals series.
Warren Gatland will name his 30-man World Cup squad on August 22.