The 22-year-old, on loan at Villa Park with a conditional obligation to buy from Chelsea, replaced Emiliano Buendía in the 75th minute as Unai Emery's side searched for a first win of the season at the MKM Stadium.
Garnacho struggled to make any impact in the time available, failing to register a shot, create a chance, complete a cross or make more than nine passes across his 15 minutes plus stoppage time on the pitch.
Former Aston Villa and Liverpool full-back Stephen Warnock was scathing of the winger's cameo on commentary duty, grouping him with fellow substitute Tammy Abraham, who was introduced in place of Nicolás Jackson in the 85th minute.
"You may as well take off Alejandro Garnacho and Tammy Abraham. They have done nothing since they have come on the pitch. Garnacho going down. He looks completely disinterested," Warnock said. "Unai Emery will be furious with his changes."
Garnacho left Manchester United last summer after 144 appearances and 26 goals for the club, moving to Chelsea for £40m, but endured a difficult debut season in west London that prompted the Blues to sanction his exit. Villa moved to sign him on loan in July, yet he has started none of the club's opening three Premier League fixtures, kept out of the side by teenager George Hemmings, who has been one of Villa's standout performers this season.
Asked after the Hull match what had allowed Hemmings to establish himself ahead of a more expensive, more experienced arrival, Emery made clear the gap in readiness between the two players.
"George, he worked last year the whole year with us and played minutes. He is performing fantastic and getting better," Emery said. "He created one assist for Buendía. He is working in the intensity and kilometres we need. He is doing it. I think he is a player now, clearly, to be in the Premier League.
"Then, we have other players who joined who need to adapt to us in training, playing matches, and Garnacho is in this way. We need their performance to try to add things, working and getting numbers. Garnacho is in this way. How he is responding, George, is really fantastic."