Aguero replaced the injured Gabriel Jesus early on in City's 2-0 win at Bournemouth in the Premier League on Monday.
It marked the third consecutive league game in which Aguero has come off the bench, leading to questions over the Argentina international's future.
But Guardiola said the form of Jesus, Raheem Sterling and Leroy Sane had made it tough for Aguero, who he talked up as still being vital to City.
"Of course we need all the players, the strikers especially. I know how important is Sergio. I did yesterday, the day before, the last week, the last month," Guardiola said.
"He's an important player that in the last games he didn't play because the three guys in front were amazing. Gabriel was outstanding and Leroy and Rash too, so that's why they played.
"But the [FA] Cup is there next week and Champions League is there, Premier League is still there.
"To qualify for the Champions League next season will be a tough, tough battle for everybody.
"So, that's why of course we need the contribution from Sergio, he's so important for us."
Aguero has scored 11 league goals in 19 games this season, and Jesus' foot injury may provide him with an opportunity.
City, who are second in the table, face Huddersfield Town in the FA Cup on Saturday before hosting Monaco in the UEFA Champions League.