Ansu, 18, has been out since November with a knee injury but returned as a second half substitute against Levante to add Barca's third goal in stoppage time.
Two quick-fire goals in the opening 15 minutes had earlier given Barca complete control of the encounter. Memphis Depay won and converted a penalty and Luuk de Jong scored his first goal for his new club after good work from Sergino Dest.
The win, which keeps Barca unbeaten through their first six LaLiga games, eases some of the pressure on coach Ronald Koeman, who watched this match from the stands as he served a touchline ban.
Barca are going through a difficult period, but they have plenty to be excited about. Ansu, wearing Lionel Messi's old No. 10 shirt, will win the headlines here. His goal was the injection of positivity Camp Nou needed. It was also an emotional moment that he celebrated with the team's doctors and his family in the stands. His dad was in tears.
It wasn't just about Ansu, though. For large parts of the game Gavi, a 17-year-old midfielder, had been the standout performer. Another young midfielder, Nico Gonzalez, 19, also impressed on his first senior start for the club.
There weren't really any from this dominant performance, which also came on the back of Atletico Madrid and Real Madrid dropping points this weekend. Levante didn't even muster a shot until the final 10 minutes, though, but it should be remembered Barca were facing a side without a win in the league since April.
Manager rating out of 10
6 -- Koeman was sent off at Cadiz on Thursday so wasn't on the bench here. However, he got his team selection spot on. He was repaid for putting his faith in youngsters Nico and Gavi, with a loosely 4-2-3-1 formation yielding Barca's best pressing performance in months.
Player ratings (1-10; 10 = best, players introduced after 70 minutes get no rating)
DF Oscar Mingueza, 7 -- Another solid display at right-back. Played one brilliant pass over the top to set up a chance for Gavi.
DF Gerard Pique, 7 -- The old head in the back four. At 34, he was playing alongside a 22-year-old and two 20-year-olds, but his experience is important in that sense.
DF Eric Garcia, 7 -- A steady performance from the summer signing, who read the game well and passed out from the back well.
DF Sergino Dest, 8 -- The United States international produced his best performance of the season. Was electric on the left, setting up De Jong's goal and creating a sting of chances throughout the game.
MF Sergio Busquets, 7 -- Looked good alongside Nico and even found time to display his trademark drag backs, much to the entertaining of the 35,000 crowd.
MF Nico Gonzalez, 7 -- Looked wobbly early on but grew into the match on his full debut. He is strong, reads the game well and keeps it simple. He will have plenty more chances this season.
MF Gavi, 8 -- This will probably be copied and pasted this season, but it was a delightful display from a 17-year-old who belies his years. Knows where to be and when, uses his body well and is brilliant on the ball. Has the makings of a star.
MF Philippe Coutinho, 6 -- Still feeling his way back into form after a nine-month injury. These minutes will do him good and he linked well with those around him in his preferred central role.
FW Memphis Depay, 8 -- A menace all afternoon for Levante. Wiggled his way out of all sorts of problems, winning and scoring the penalty which opened the scoring. The only downside was he didn't score another given the chances he had.
FW Luuk de Jong, 6 -- A good game from the classic No. 9. Took his goal well and worked really hard.
MF Riqui Puig, 6 -- Almost had an instant impact, setting up a chance for Memphis before slaloming into the box only to delay getting his shot off.
FW Ansu Fati, 8 -- What an incredible young man. To come back after over 10 months out and play like he did for 15 minutes was outstanding. Although perhaps, given the impact he had as a 16 year old in LaLiga and the Champions League, it should not be surprising that he didn't feel the pressure.
FW Yusuf Demir, NR -- Replaced Gavi for the final 10 minutes or so.
DF Clement Lenglet, NR -- Came on for Dest in stoppage time.