Career
Keller has played professionally for VfB Stuttgart, Turn- und Sportverein 1860 München, VfL Wolfsburg, 1. Football Club Köln and Eintracht Frankfurt. On 13 October 2010, Keller became interim manager of VfB Stuttgart until a permanent appointment had been foundation
He was replaced by Bruno Labbadia after two months in charge of the team on 12 December 2010.
On 16 December 2012, Keller was promoted from his position as the U17 coach to be the new head coach. His contract for Schalke 04 lasts until the end of the season.
On 10 May 2013, Keller"s contract with Schalke 04 was extented for 2 more seasons. After only two wins in 10 matches in the 2014-2015 season, Keller was sacked on 7 October 2014 and succeeded by Roberto Di Matteo as head coach.