Atletico Madrid were 2-0 up away to Las Palmas after three minutes thanks to goals from Angel Correa and Yannick Carasco.
Thomas Partey completed the rout after the home side's striker Jonathan Viera had a penalty saved by Jan Oblak.
Barcelona was still without Ousmane Dembele, who was announced as Neymar's replacement on Friday, and it also couldn't count on the injured Luis Suarez. We're at a point where Barca are spoken about a lot and it seems like that can have an effect, and the team showed that that isn't the case.
Is it the same old names we've been discussing all summer or are they going to spring a surprise on us all?
Alaves, who were beaten by Barca in last season's Copa Del Rey final, brought on Enzo, the son of Real Madrid boss Zinedine Zidane, for his debut in the second half. For the second week in a row, Barca were far from flawless, but after a shaky pre-season, the club will be more than happy with the six points.
Ernesto Valverde suggested Barcelona will make more signings before the transfer window closes after watching his side make it two wins from two in La Liga at Alaves on Saturday.
Belgium worldwide Carrasco doubled the visiting side's advantage with an arrowed shot which flew past goalkeeper Leandro Chichizola and into the far corner.
The second goal, coming 11 minutes later as Paco Alcacer finally did something for the club after a disgusting debut with an assist, had more to do with embarrassing defending by the home side than anything brilliant from Messi.
It was the second consecutive loss for Las Palmas. "They sat very deep and it was hard to find spaces".
"We helped ourselves by taking an early advantage and then it was a totally different game".
Nonetheless, the second half began with Jordi Alba teeing up the Argentine, who smashed the ball into the roof of Pacheco's net via a deflection.