Title-chasing City were held to a 2-2 home draw on Wednesday, a result that leaves them six points behind leaders Liverpool with a game in hand.
The Chilean said: "Of course it was a disappointment because we always want to win every game but like I said before that game, playing against relegation teams is not easy they play 100 per cent every game, Sunderland did it here very well."
The City boss says he expects there to be plenty more twists and turns before the destiny of the Premier League title is finally decided.
"I am absolutely sure teams will drop more points. Liverpool, Chelsea could drop the same way we dropped in the last game," he said.
"So we have to play for another 15 points, we have to try and win the most amount of points, at the end of the season we will see which team have won the title.
"We are trying to prepare for next season but first we must try to finish this season. It's important to have all our minds - player minds, the manager's mind and everyone here at the club on trying to finish the season in the best way."
City face West Brom at the Etihad, live on Sky Sports 1HD, and Pellegrini is expecting his team to improve as they chase their second Premier League title in three years.
He said: "All the games are different, performance in every game is very important and I hope that we are going to improve our performance against West Brom."
City are sweating on the fitness of David Silva and Jesus Navas, who both had to sit out the Sunderland game due to ankle injuries.
"We continue with the same situation with Yaya Toure, David Silva and Jesus Navas," said Pellegrini. "The three of them are injured for the moment."
Pellegrini has revealed that injured midfielder Toure is improving but is not expected back soon.
The Ivory Coast international, who has been nominated for the PFA Player of the Year award, suffered a groin injury in the 3-2 defeat at Liverpool.
Toure was initially all-but ruled out for the season but Pellegrini said: "It is improving but it's a muscle injury so it's difficult for him to be back so soon."