The 17-year-old forward had netted the first two goals of Ronald Koeman’s reign as the Catalans thrashed Villarreal 4-0 on Sunday and was on target again as he poked home with the outside of his right foot from just inside the area after 11 minutes at Balaidos.
That gave Barca the edge at a ground where they have struggled in recent years and in the pouring rain, a frequent sight in Galicia.
Later in the half, Gerard Pique was shown a red card for hauling down last man Denis Suarez, but the former Barca midfielder was offside and the Catalan was handed a reprieve.
But shortly before the interval, Barca defender Clement Lenglet was somewhat harshly sent off for a second bookable offence and Koeman sent on centre-back Ronald Araujo for Antoine Griezmann.
Despite their numerical disadvantage, Barca remained the better side and Koeman’s men continued to press in the second half and went 2-0 up after 51 minutes when Messi ran into the area and saw his cross deflected in by Lucas Olaza for an own goal.
Messi later thought he had scored himself as he dispatched a rebound after Philippe Coutinho had hit the post, but the Argentine was marginally offside and it did not count.
Celta hit the bar late in the game through substitute Miguel Baeza’s deflected shot, but the Galicians rarely looked like scoring and Barcelona made it 3-0 deep into added time as Sergi Roberto smashed a shoot into the roof of the net following more super play by Messi.
Koeman’s side made it back-to-back wins and have scored seven goals and conceded none in the Dutchman’s opening two games in charge.
For Celta, meanwhile, it was a first defeat of the season after two draws and a win in their first three fixtures in 2020-21.