The Spurs midfielder joined the Bundesliga side on loan during Deadline Day, with a view to a permanent deal.
That transfer ended a one-and-a-half year stay at White Hart Lane, despite Holtby initially signing a four-and-a-half year deal with the north London side.
The German didn't enjoy the most productive of stays at Tottenham, having been loaned to London rivals Fulham last season.
But many fans feel the 23-year-old should have stayed at the Premier League side, while Walker is sad to see him go.
After the switch was confirmed, the full-back took to Twitter to send a good-luck message to his 'true friend.'
Spurs won their opening two Premier League clashes this season, beating West Ham and Queens Park Rangers to go top of the table on goal difference.
A 3-0 defeat to Liverpool brought the north London side back down to Earth on Sunday, however, with Mauricio Pochettino's side due to travel to Sunderland after the international break, before hosting West Brom.
Going in the other direction on Deadline Day was Benjamin Stambouli, who joined Tottenham from Montpellier for an undisclosed fee.