City were regarded as the favourites to win the trophy by many bookmakers prior to Wednesday's match, but with manager Pep Guardiola banned from the touchline the Premier League holders slumped to their first defeat of the season.
First-half goals from Maxwel Cornet and Nabil Fekir ultimately proved to be the difference as the visitors picked up a famous victory, but Stones was eager to quickly turn his focus to the future and insists that there is still plenty of time to make up for the shock result.
"Really disappointing. To concede two goals like we did is very frustrating. We came in at half time a bit deflated. We picked ourselves up and played a better second half but it was frustrating," Stones told BT Sport.
"It is the first game. Still early days. We put pressure on ourselves. There is no outside pressure. We know our strengths and weaknesses. We go again in the next game. There are still a lot to play and we should be positive about that."
The defeat means that Man City are now the first English team to ever lose four successive Champions League matches.