The whistling from a section of the support wasn't aggressive, but it could be heard just before halftime. Messi collected the ball on 43 minutes and ignoring teammates, went for goal with his head down.
Upon losing possession, the whistling could be heard around the stadium - almost in reaction to Diego Maradona's comments last week that Messi can be selfish in games.
Asked about the incident at the end of Barca's 3-1 win, coach Pep Guardiola said: "Messi has been ten days away from us and is still adjusting to the season schedule. He will gradually get better."