Franco Di Santo stunned the Allianz Arena by equalising in the 29th minute, burying his shot after Leon Goretzka mis-kicked from Breel Embolo's cross.
"It was bitter to concede the goals, especially the second", admitted Faehrmann.
Well earned. It's good to have you back.
Mueller said it was a lucky strike. Bayern have won 11 and drawn four of their past 15 matches against Schalke, whose last win over the Bavarians was in the 2010/11 season.
Plenty has changed since he last started a game for Dortmund.
Eight months ago the 28-year-old partnered Ousmane Dembele and Pierre-Emerick Aubameyang up front. "I am happy to be part of this team", he said. As promised, the Belgian striker celebrated with a somersault.
Dortmund lived dangerously with Hamburg threatening an equalizer, but substitute Mario Gotze sealed the win in injury time after Andre Schurrle played him in to lift the ball over the onrushing goalkeeper.
Dortmund's injury-prone Germany winger Marco Reus returned after 259 days sidelined by a torn knee ligament.
Hannover ended Freiburg's nine-game unbeaten run with a 2-1 win at home to consolidate its midtable spot, level on points with Hoffenheim and Augsburg.
Eintracht Frankfurt stormed up to fourth with a 4-2 thrashing of bottom side Cologne.
Bayer Leverkusen missed a chance to go second when they lost 2-0 at home to Hertha Berlin and remain on 35 points.
Freiburg lost for the first time in nine league games, crashing to a 2-1 defeat at Hanover 96, whose midfielder Felix Klaus scored the victor.