Lampard left Stamford Bridge in the summer after 13 years with the club and signed a deal for future MLS side New York City FC, before being loaned to Manuel Pellegrini's side for six months.
The midfielder netted his first goal for the Citizens as he struck in the 86th minute to earn them a point against his old club.
Mourinho told reporters: "Frank Lampard is a Manchester City player. I don't believe in stories of passion and heart, maybe I am too pragmatic.
"When he decided to go to a direct competitor, then love stories are over. He did his job as a professional.
"He used to play for Chelsea and Luis Suarez used to play for Liverpool. Now Suarez plays for Barcelona. For me there is no drama."
Chelsea are top of the Premier League after five matches this season.