"Diego is not in the best condition, but he will start the game," Mourinho told reporters at Chelsea's Cobham training ground, south of London.
"He plays this game, he cannot play against Bolton. It's as simple as that."
When asked if the player would benefit from two weeks' complete rest, Portuguese boss Mourinho added: "Yes."
Mourinho and his players face the prospect of a reunion this weekend with Frank Lampard, who unexpectedly joined City on loan from New York City before the start of the season.
Lampard agreed terms with the United States club after leaving Chelsea as a free agent at the end of last season following a 13-year stay at Stamford Bridge before making a temporary switch to City ahead of the 2015 Major League Soccer season.
Mourinho said he never expected to face Lampard in these circumstances after the 36-year-old midfielder left Chelsea.
"It's football. If you'd said that four or five years ago, I would have said it was strange," said Mourinho.
"Even a few months ago, when he signed for the New York team, I was not expecting this. But it was his decision. It's life."
- AAP