The 20-year-old started at right-back in Saturday's 6-1 victory over Alavés at the Spotify Camp Nou, becoming the latest product of La Masia to receive an opportunity under the German manager.
Flick was forced to make several adjustments because of six unavailable players, including four defenders. Rather than turn exclusively to established first-team options, he handed Fariñas his opportunity while deploying João Cancelo on the opposite flank.
The decision meant Alejandro Balde started on the bench, despite being available for selection.
Fariñas, who wears the number four shirt, joined Barcelona in 2018 after coming through Villarreal's youth system. Primarily developed as a midfielder, he has demonstrated his versatility during his progression through the club's academy.
His ability to operate at right-back had already attracted Flick's attention during pre-season, when he featured in the 2-2 friendly draw against Birmingham City.
The teenager looked comfortable on his competitive debut, showing confidence in possession and composure when Barcelona were required to defend.
He regularly offered himself as an outlet, linked effectively with Lamine Yamal and combined particularly well with Pau Cubarsí and Rodri. He also showed an appetite to advance into attacking areas rather than simply concentrating on his defensive responsibilities.
His passing was another encouraging aspect of the performance. Fariñas completed 106 passes, failing to find a team-mate on only four occasions, and remained on the pitch for the full 90 minutes.
The performance was particularly impressive given the circumstances of his introduction. Barcelona were attempting to respond to consecutive defeats against Paris Saint-Germain and Real Madrid, making the match an important opportunity to restore confidence.
Fariñas was not the only academy player attracting attention, with Flick also including Guillem Víctor and Ebrima Tunkara in the matchday squad for the first time. Neither made their senior debut, but their inclusion reflected the manager's willingness to involve the club's emerging talent.
Barcelona eventually produced an emphatic response, with Raphinha and Lamine Yamal both scoring twice, while Fermín López and Anthony Gordon also found the net.
The victory kept Barcelona at the top of LaLiga and provided some relief after their recent setbacks.
For Fariñas, however, the occasion was a significant personal milestone. Having progressed through the club's youth system since arriving as an under-14 player, he has now taken the next step towards establishing himself in the senior squad.
His versatility could prove particularly valuable as Barcelona navigate their injury problems, while his assured performance gives Flick another option to consider when his established defenders return.