Borussia Dortmund have been dealt an injury blow as Mats Hummels could miss their Champions League quarter-final clashes after being ruled out for a month with an ankle injury pickedup as Jurgen Klopp's side were defeated 2-1 at Schalke in the Revier derby, and the Germany international had to be withdrawn at half-time due to a problem picked up in the first 45 minutes.