The former Liverpool midfielder was part of a Barca midfield that was over-run by Hercules as they minnows shocked the champs 2-0.
Mascherano was replaced at half-time by Xavi as Barca looked to adopt a more attacking approach in the second period, but Guardiola was pleased with his new player.
"Mascherano played really well - I just felt we needed to change things," he said.
Mascherano, meanwhile, was disappointed to lose his first match as a Barca player.
"It was a great surprise to start the match and my team-mates helped me a lot. I felt comfortable, but it's a bitter feeling to lose. There's not much more I can say - Hercules played a virtually perfect game," he said after the match.