Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: Jurgen Klopp wants Borussia Dortmund's reserve team manager David Wagner, the former USA international, to join his Liverpool staff, Sportsmail understands. The pair formed a friendship playing alongside each other at Mainz between 1991 to 1995 and were reunited at Dortmund when Wagner took charge of the club's Under-21 side in July 2011. Klopp is looking to shape his own backroom team at Liverpool, with Peter Krawietz, his assistant at the Wesfalenstadion, and Zeljko Buvac, another established associate, moving to Anfield. Liverpool assistants Gary McAllister and Sean O'Driscoll, appointed in the summer, will leave the club, so too Chris Davies, head of opposition analysis, and Glen Driscoll, head of performance.David Wagner played alongside Jurgen Klopp at Mainz in the early 1990sWagner was reunited with the manager at Borussia Dortmund in 2011Klopp wants the Dortmund reserve team boss to join his Liverpool staffAssociates Peter Krawietz and Zeljko Buvac will be included in backroomLiverpool assistants Gary McAllister and Sean O'Driscoll will leave Anfield 
 Question:_ had a nomadic playing career, appearing for eight clubs including Schalke.

David Wagner