However, Stevens resigned on Wednesday after less than four months in the role due to heart problems - and the club have decided under-19 coach Nagelsmann will step up to the senior role early, on a three-year contract.
"Together with Julian, we again carefully weighed up all the risks we considered in the autumn and have now decided the best way is for him to start earlier.
"He is keen and he can also give the team fresh impetus."
Nagelsmann, who the club expect will pass his senior coaching examinations soon, said: "It's a big challenge, of course, made harder by the current points situation.
"But I am looking forward to working with the team."
Hoffenheim are currently joint bottom of the Bundesliga table with Hannover, five points adrift of third from bottom Werder Bremen - and Nagelsmann's first match in charge will be at Bremen on Saturday.
- Telegraph