There has been speculation that Keita's move could be brought forward to the January window but Mintzlaff says that won't be happening.
"Neither Naby nor his agent have made the request to leave the club in the winter," he told Bild.
"There are contracts, and they say that he will be a Liverpool employee from July 1 onwards."
Mintzlaff has no doubts about the midfielder's commitment.
"It's not Naby's disposition (to save himself). Our pros have character and are ambitious," Mintzlaff added.
"I am not worried. In 2018 I expect everyone to step up with maximum mentality, force and character. That's what makes us strong, and that's what we need for a successful second half of the season.
"We want to achieve a lot, want to return to Europe and would love to again qualify for Champions League."