The 25-year-old player was forced to miss the recent match against Arsenal after undergoing a scan for a groin injury on Sunday morning.
Howe refused to confirm whether Isak will be available for the crucial match against Everton, which will determine the team's chances of qualifying for the Champions League.
In a statement after the game, Howe said that it's still unclear if Isak will be ready to play.
Isak had experienced some pain during training, but had felt fine beforehand, so he underwent a precautionary scan, which revealed the injury.
Howe noted that Isak has had similar groin injuries in the past and will need to be monitored in the coming days to determine the extent of the damage.