In the meantime, Ralf Rangnick will combine his role as Leipzig's director of football with first-team coaching duties in 2018-19.
The 31-year-old Nagelsmann took over Hoffenheim in February 2016, and led the club to a fourth-place finish in 2016-17 which saw them make the Champions League qualification stage.
Though elimination to Liverpool followed, Nagelsmann will guide Hoffenheim to the group stages this season in his swan song with the club after a third-place finish in 2017-18.
And the Denmark international Poulsen is excited to work with Nagelsmann when the opportunity arises in the 2019-20 season.
"Of course this is a top transfer. I've heard a lot of good from other players about him," Poulsen said via Bild.
"Julian did a super job in Hoffenheim. I am looking forward to working under him."
Interestingly, Poulsen was so busy with the World Cup with Denmark, who fell to eventual finalists Croatia on penalties in the round of 16, and his ensuing vacation that he was not aware of Rangnick taking over for this season.
"The takeover of Ralf Rangnick as a coach with us, I have not noticed," Poulsen admitted.
"I had my phone off on vacation, did not find out until the return flight a few days ago."