Martin Petrov was a constant thorn in the West Ham side and City had strong shouts for a first-half penalty when he went down under a challenge from Anton Ferdinand.
"I think it was a penalty," said Eriksson.
"I think we were unlucky. We will have it at home, hopefully.
"Everything is still open. I hoped to win this game but a draw is okay for me. It was a good performance. We defended very well for 90 minutes.
"We tried to create occasions to score and maybe we should have scored in the first half."
While some Premier League clubs opted to rest key players for the third round, both West Ham and City fielded full-strength sides.
Eriksson said: "If you are in the Champions League and are fighting to win the Premier League, then I can understand it.
"If you want to be competitive all over the place you have to almost have two teams and rotate players.
"I did it with Lazio when we were playing in the Champions League and trying to win the Italian league and the Italian Cup. You have to rotate players. Every time we did it we never lost.
"But we are not playing in Europe, we were knocked out of the Carling Cup so we will do everything we can in the league and the FA Cup."
West Ham created chances of their own but generally lacked any cutting edge up front.
The Hammers felt they deserved a penalty of their own, claiming a handball from Vedran Corluka in the dying seconds.
Boss Alan Curbishley said: "I am delighted what our players have done defensively but we just need a bit more going the other way.
"I can't ask any more from the players. Everyone connected with the club realises the situation we are in and we will just keep going."