With time running out Barcelona coach Ernesto Valverde introduced Paulinho in the 78th minute.
Barcelona preserved their 100 percent start to the season as Paulinho's late victor secured a 2-1 victory over Getafe in La Liga on Saturday.
But another of Valverde's subs, Denis Suarez, struck for the first time since January as the visitors battled back in the second half, before Paulinho grabbed the victor just as it looked like they would have to settle for a draw.
Dembele, Barcelona's record signing, had started against Juventus in the club's 3-0 Champions League win on Tuesday.
Getafe remain on four points and have now gone 10 LaLiga matches without a home win, despite a courageous performance. Real Sociedad hosts defending champion Real Madrid on Sunday.
Getafe, which has won only one of its four games, led through Japanese midfielder Gaku Shibasaki just before halftime.
Cardinals' Johnson has dislocated wrist: coach
Johnson will undergo an MRI on Monday, and the results of that test will determine how much time he will miss. Johnson came off the field and said he could play, Arians said, but later couldn't play and left the game.
Signed in the summer transfer window for a club-record £96.8m to replace Neymar, who joined Paris St-Germain, Dembele was making his first league start for Barcelona.
Messi nearly found a quick-fire equaliser, his free kick curling towards Guaita's top-left corner, but the goalkeeper managed to tip it over.
Player ratingsBarcelona: Marc-Andre ter Stegen 6, Sergi Roberto 7, Gerard Pique 6, Samuel Umtiti 6, Jordi Alba 6, Sergio Busquets 6, Ivan Rakitic 6, Andres Iniesta 5, Ousmane Dembele 5, Luis Suarez 6, Lionel Messi 7.
GOOOALLLLL Denis Suárez equalises!
Messi had a free-kick brilliantly saved by Guaita and Getafe could have gained at least a point when Mauro Arambarri's deflected shot struck the foot of the post.
Barcelona will attempt to make it five straight La Liga wins at the start of the new season when they welcome Eibar on Tuesday night, while next up for Getafe is a trip to Celta Vigo on Thursday.
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