The 2012 Premier League champions were on course to take a valuable point at Stamford Bridge before a defensive mix-up between goalkeeper Joe Hart and Matija Nastasic resulted in Fernando Torres netting a 90th-minute winner.
Demichelis could do nothing to stop the Spaniard as the Argentina defender made a belated first appearance for City following a knee injury sustained soon after signing at the beginning of September.
"It's a weird feeling," Demichelis told mcfc.co.uk.
"I'm sad because we lost the match in the last minute.
"(But) it's great to be back on the pitch; I'm very happy to make my debut for City.
"I'm disappointed because we didn't get the three points. I think we showed good character, especially second half.
"It's not easy to come to Stamford Bridge. We played really well in the second half and we showed how we want to play."
The calamitous end heaped further scrutiny on England goalkeeper Hart, who has made a number of high-profile errors already this season.
Following the match City boss Manuel Pellegrini refused to publicly back the 26-year-old, who could now be dropped in favour of Costel Pantilimon, the Romania goalkeeper.
Next City take on Newcastle in the Capital One Cup fourth round on Wednesday, with Demichelis confident of a positive result if the Stamford Bridge performance is repeated.
"If we keep playing like this we're going to get a lot of good results," he said.
Demichelis played under Pellegrini at Malaga and joined the Chilean at the Etihad Stadium soon after signing for Atletico Madrid, without making an appearance.
"I'm very happy in Manchester," Demichelis said.
"I had a difficult start because I picked up an injury. It's not the best way to start when you join a new club.
"I worked really, really hard to be back soon. I'm a player with a lot of experience and I know injuries are a part of the game.
"I'm very happy to be back with my colleagues."
- Sports & Recreation
- Matija Nastasic
- Fernando Torres
- Joe Hart