With the market shut, Guardiola admits his squad his short of options in attack.
Alvarez was sold to Atletico Madrid for a fee rising to £81m.
Guardiola said, "I'm happy with the squad. We didn't expect Julian was leaving, although he had conversations a long time ago with other clubs and his agent, so it could happen and in the end it happened.
"We saw what we have, if we have many, many injuries it will be a problem, but in a few months Oscar Bobb will be back (from injury) and Phil (Foden) is in that position, (James) McAtee can play in that position, (Ilkay) Gundogan can play in that position, Bernardo…
"They are different players to Erling (Haaland) of course. Maybe it is a mistake (letting Alvarez leave), I don't know but I don't like leaving players without playing for a long time. When everyone is involved with a chance to play the performance of the teams is always better.
"I know the group and how serious and professional they are and their commitment in every single game. We are pretty sure it is better to wait."