A series of good results and performances under new coach Julio Cesar Falcioni in friendlies in January had led to Boca being named as the team to beat after some good signings and the return to the side of Juan Roman Riquelme.
Riquelme missed virtually all of the Apertura championship in the first half of the season following knee surgery last May.
"When you lose you can't say it's bad luck, there was good work by our adversaries. We left spaces and they hurt us a lot," Falcioni told the post match news conference.
"We had four or five goal chances under the crossbar, we couldn't put them away, that's all... we deserved more, not to lose by this result.
"Whether or not we are (title) candidates, this defeat doesn't take anything away from us."
Boca's arch-rivals River Plate were held to a 0-0 draw at Tigre while Racing Club won 1-0 away to All Boys.
Title holders Estudiantes opened the championship with a 2-1 home victory over Newell's Old Boys and Velez Sarsfield, runners-up in the Apertura, came from two goals down for a 2-2 draw at Independiente on Friday.
