Aguero has 11 goals in nine games for City this season, but has not found the net in each of the Premier League leaders' past three winless outings.
The Argentina international came off the bench to miss a penalty against Everton on Sunday (AEDT), where his omission was widely viewed as a tactic to preserve him to take on Barcelona in UEFA Champions League Group C.
But Guardiola said he named a side without a recognised striker in an attempt to prevent Barcelona from establishing its customary midfield dominance.
“I want more midfield players, that is why," he told BT Sport when asked about benching Aguero. "We come here to try to win the game.
“I spent almost all my career here but I am not here to have fun or for a holiday. I am here to do the best job possible.”
Guardiola conceded the game was "over" when his side was reduced to 10 men in the 4-0 Champions League defeat to his former club Barcelona.
City asked questions of their host after Messi's 17th-minute opener at Camp Nou, but Claudio Bravo's botched 53rd-minute clearance and subsequent handball to prevent a Luis Suarez goal left them with a mountain to climb.
"At that level it's hard, but until 10 against 11 the game was open and we competed against a very good team with huge personality," visiting manager Guardiola told BT Sport after a painful outing at the ground where he made his name," he said.
"We created enough chance to score goals. After [the red card] the game was over.