Shkodran Mustafi has moved to Schalke from Arsenal as a replacement for centre-back Ozan Kabak, who has joined Liverpool.
The Germany international moved to the Emirates in the summer of 2016 but has frequently struggled for form after an encouraging start to his career in north London.
He was part of two FA Cup winning teams, but on both occasions was missing for the final due to injuries, which have affected his progress. It is reported that Arsenal terminated his contract in order to progress the move.
Opinion: Arsenal have had the perfect transfer window
This season, he has been far down the pecking order, with Rob Holding generally playing at the heart of defence alongside either Gabriel Magalhaes or David Luiz.
Calum Chambers is still at the club but has only recently returned from an anterior cruciate ligament injury, while there are high hopes for William Saliba, who is on loan at Nice.
The 2014 World Cup winner has never played top-flight football in his home country, having moved to the Everton academy before featuring for Sampdoria and Valencia prior to his move to the Premier League.
His move to Schalke will see him plunged into a struggling team, with Christian Gross' men rooted to the foot of the Bundesliga table after an appalling start to the season which has featured only one win just past the midway point.
His move to the Veltins Arena paved the way for Ozan Kabak to move to Liverpool. The 20-year-old Turk has been one of the few bright spots during Schalke's season and the Premier League champions are in dire need of reinforcements at the back due to injuries to both Virgil van Dijk and Joe Gomez.
A true trailblazer – re-live Serena's best Australian Open moments
It also reunited him with Sead Kolasinac, who left Arsenal earlier in the window for his former club on loan until the end of the season.
Our view - Mustafi had to be a part of Arsenal clear-out
There was much fanfare around Shkodran Mustafi's move to Arsenal all those years ago, and it is easy to forget that he was initially a great success, forming a successful partnership at the heart of defence with Laurent Koscielny dubbed 'Kostafi' by fans. It was not until the end of January 2017 that he lost a Premier League game.
But, truth be told, he never recovered from the concussion which forced him to miss the tail end of the season and that season's FA Cup final. And with Arsenal slipping down the table - they were second in the campaign before he arrived and eighth last term - he became the object of ire from the stands, which affected him greatly by his own admission. Booing him during a pre-season friendly was hardly an edifying episode.
There have been some good memories - a goal and an exemplary performance against Harry Kane in a November 2017 win over Tottenham was a particular highlight. But his Arsenal career will be defined by the sort of gaffes seen in the 3-2 home loss to Crystal Palace in April 2019 which all but saw them miss out of Champions League qualification.
With Arsenal getting the likes of Sokratis, Mesut Ozil and Sead Kolasinac off the wage bill this January, it would only have been right to include him in the clear-out.
Maitland-Niles to Leicester or WBA, left-backs to Arsenal - Deals to look out for on Deadline Day
Liverpool interested in signing Mustafi - reports