Barcelona missed a chance to move within a point of leaders Sevilla after the Spanish champions were held 1-1 by Atletico Madrid at the Nou Camp.
Aguero joined Atletico from Independiente in the summer
Ronaldinho bent a free-kick past goalkeeper Leo Franco just before half-time to give Barcelona the lead.
But Atletico equalised on the hour when their young Argentine striker Sergio Aguero scored with a flicked finish.
The draw left Barcelona three points behind Sevilla, though the leaders have played a game more than the champions.
Atletico have become something of a bogey side for Barcelona over the last few years, having won three and drawn twice of their last five meetings.
They have also been the only team to have beaten Barcelona in their own stadium as the Catalans have won back-to-back league titles, with Atletico striker Fernando Torres scoring six times in the four games before Thursday's encounter.
The Barcelona defence prevented Torres from adding to that impressive tally, but they proved less successful in keeping track of his striking partner Aguero.
Atletico's 18-year-old Argentine forward equalised with a superbly taken goal, timing his run to perfection to collect Peter Luccin's pass, before stabbing the ball past Victor Valdes.
Barcelona had dominated the first half, with Deco going close with a raking shot, while Xavi forced two reaction saves from Franco.
Atletico had adopted a defensive strategy in that opening period, but they were more ambitious after the interval and were rewarded with Aguero's opportunistic strike.