City believed they were in pole position to land Lionel Messi after the Barcelona legend declared his intention to quit Camp Nou.
But the Argentine was ultimately forced to stay with the Catalan giants – leaving Guardiola with no other options.
Now City have been hit with a striking crisis, with both Aguero and Gabriel Jesus out injured.
Aguero already missed the start of the season as he recovered from a knee operation – and suffered a muscle strain in Saturday’s draw with West Ham.
With Jesus also out with a thigh problem as City prepare to face Marseille on Tuesday, Guardiola revealed his plans to strengthen up front during the last window.
“We thought about it, another type of striker, but we couldn’t do it,” he said. “The club did what it could. They want to reinvest in the team. Our team is so young.
“When you decide to buy a striker, they need to be on Gabriel and Sergio’s level and you cannot afford it. That’s the reality.
“I’m not saying the club didn’t want to do it, they wanted to make as strong a team as possible, but we thought Sergio is coming back and we didn’t expect Gabriel’s injury.”
Aguero returned to action in City’s win against Arsenal earlier this month, with Guardiola admitting he knew there was a danger of the 32-year-old breaking down.
“If you’re four or five months out with a knee problem you have the risk of injury when you come back,” he said. “We tried to handle it the best as possible, 50-55 minutes in the games, but it was not enough to maintain it and get him through the games.
“We knew he was not in the best condition against Arsenal, but without strikers that we had, it was important that he played 60 minutes, but we tried to rest him when the muscle started to be tired.
“Unfortunately, one action in the first half (against West Ham) was unlucky because he wanted to stop a long shot at West Ham and it happened. We have to be calm and hope he recovers as quickly as possible.”