In what could prove a top four six-pointer, Forest won 1-0 at the City Ground thanks to Callum Hudson-Odoi's 83rd minute strike.
The result leaves Forest in third place, four points ahead of City, which sit fourth a point ahead of fifth place Chelsea, which have played a game less.
Guardiola remarked afterwards: "We have 10 games to qualify, we have to win games to qualify and we move on to the next one.
"We knew the game would be difficult. Every season the Premier League gets better and better. We have 10 finals and the next one we start."
On Forest, he also stated: "A tight game, we controlled transitions, we had chances to shoot but were not precise. In the second half there we transitions and in a transition they scored.
"They defended really well. We had chances but in last moment we could not do it."
Guardiola added, "It is not easy to play against them. That is why they are where they are. We had three or four chances in the first half. Second half it was a bit more tight. We concede one chance in the near post, so in the end the balance was for them.
"We struggled to break the lines and find spaces."