The Spain midfielder, 27, who joined from Barcelona in the summer, swept in the opener from Eden Hazard’s pass after an apparent foul on Max Meyer.
Didier Drogba shot wide when through before striker Klaas-Jan Huntelaar drove in an equaliser after a counter.
The introduction of Diego Costa and Loic Remy could not swing the three points Chelsea’s way
Loic Remy had a shot cleared off the line as Schalke held on for a deserved 1-1 draw.
Bayern Munich’s Jerome Boateng scored a stunning last-minute winner to deny Manchester City a point in their opening Champions League group game.
Former City defender Boateng lashed a fierce shot across Joe Hart after the visitors had failed to clear a corner.
Hart had earlier kept City in the match with a string of saves from Thomas Muller, Mario Gotze and Boateng.
City’s best chance arrived in stoppage-time when Sergio Aguero beat Dante to the ball but poked narrowly wide.