Both players were named in Hansi Flick's starting lineup as Die Mannschaft secured a routine 2-0 friendly victory against Peru at the Mewa Arena on Saturday.
Havertz, who operated in an advanced midfield role for 75 minutes, teed up Niclas Fullkrug for a 12th-minute opener before missing a second-half penalty midway through the second half.
Schlotterbeck, meanwhile, partnered Matthias Ginter at centre-back before coming off in the 86th minute with an apparent thigh injury.
Flick provided a reassuring update on Schlotterbeck's condition after the match, telling reporters: "I was just with him and he said that everything is fine. He had a little cramp, but now everything is okay."
However, Schlotterbeck's injury now appears to be worse than first feared as Germany have confirmed that the 23-year-old will miss Tuesday's friendly clash against Belgium at RheinEnergieStadion.
Die Mannschaft have also confirmed that Havertz is unavailable for selection and has returned home due to illness.
Germany, who are hosting Euro 2024, have lost only one of their last 10 games on home soil in all competitions, but Flick is aware of the threat Belgium could pose on Tuesday.
"It's a different calibre of opposition. We're pleased to be playing against a team like them. It helps you to improve your game," Flick told reporters on Sunday.
"Perhaps there will be one or two changes, but we want to keep the main core the same."
Germany will be confident of success on Tuesday as they have not lost any of their last 16 meetings against Belgium, who are now managed by former RB Leipzig boss Domenico Tedesco.