Messi got back to his marauding best and scored the opening goal before Javier Mascherano saw red in the closing stages of the first half.
With Barca down to 10 men Munir stole the limelight, latching on to a ball over the top from summer signing Ivan Rakitic before deftly flicking it into the top corner with the outside of his left boot.
The teenager left the field to a standing ovation from the Nou Camp crowd just six months after he was playing for Barcelona's Under-18 side.
Messi had the final say though, shrugging off four defenders and lashing home to remind everyone of his sensational talents just after the hour-mark.
Nolito took centre stage for Celta Vigo as they eased to a 3-1 win over Getafe at Balaidos.
The winger, who was linked with a move to Everton earlier this summer, scored the opening goal in the 20th minute, before Fabian Orellana added the second after the break.
Jorge Sammir pulled one back to give the hosts something to shout about, but Joaquin Larrivey wrapped up a stylish win in the 70th minute after neat link-up play with Nolito.
Elsewhere Eibar got off to a dream start in their first ever season in the Primera Division, with midfielder Javi Lara crashing a free-kick into the net from the tightest of angles to score the only goal of the game on the stroke of half-time.
While La Real produced a strong second-half display, they could not turn their late pressure into goals.
In Sunday's late game, second-half strikes from Ikechukwu Uche and Denis Cheryshev were enough to get Villarreal's campaign off to a winning start with a 2-0 triumph at Levante.