Isak, a £125m acquisition, and Wirtz, bought for £100m, have been used sparingly this Premier League season due to ongoing fitness concerns.
The manager's comments precede a demanding fixture schedule for Liverpool, which includes matches this week against Sunderland and Leeds.
Slot also indicated that defender Joe Gomez is not ready for successive full games, and both Wirtz and Isak have recently experienced fitness setbacks.
Injured players Conor Bradley and Jeremie Frimpong are unlikely to feature against Sunderland, though Bradley might be available for the Leeds match.