The 26-year-old hasn’t featured for Arsenal since he picked up a knee injury in training after their opening day win over Man United.
Havertz had to go under the knife, but as of yet, there is no solid return date for the Germany international, and his recovery could now take longer than expected.
Speaking after Germany’s 6-0 World Cup qualifier win over Slovakia, Nagelsmann confirmed that Havertz has had a setback.
"Kai (Havertz) had a minor relapse, but overall he’s doing well,” he said.
“Jamal (Musiala) no longer has any major problems. But like Kai, he first needs to get back into rhythm with his club. They have until March to get fully fit, but they obviously need to stabilise now."
Newcastle striker Nick Woltemade has done well in Havertz’s absence, scoring four goals in his six Germany appearances since the start of the season.