"It's a little injury, a week to 10 days he will be out," Guardiola told reporters ahead of Saturday's home game against Nottingham Forest.
"(They are playing) a lot of games. Less games, less injuries. We've had a lot of games in a row with not much rest, it's just that. We demand a lot of effort and sometimes they can't sustain it.
"Jack (Grealish) was at Sheffield United with a knock on his knee - it's football. But muscular injuries mean you miss a lot of games."
Guardiola said Grealish (thigh injury) has trained and could play a few minutes while Kovacic (back) is still on the mend.
The Spanish manager also said he was placing his trust in young players to step up and praised Julian Alvarez (23) after he started the season with four goals and two assists in his last five games.
"He always has good behaviour, when he's playing or not playing, being a world champion or not. It's always the same," Guardiola said.
"The perfect dose of disappointment when he doesn't play but always ready for the opportunity. Right now with the injuries, he's had minutes and he gives us a lot of good things. That's why he's playing, but he has to continue."