Vinicius, 18, finalised a deal worth a reported €45 million (£40m/$52m) over the summer from Flamengo, after shining for the Rio de Janeiro giants as well as Brazil's Under-17s.
The forward has had to bide his time under coach Julen Lopetegui, sitting out the start of the season in favour of honing his trade at feeder club Castilla.
After three goals in three games for Castilla, however, it appears Lopetegui has been convinced of Vinicius' quality, turning to the youngster towards the end of a tense 0-0 draw against Madrid's city rivals.
Vinicius replaced Karim Benzema in the 87th minute, although he was unable to make the difference on the scoresheet as Jan Oblak turned in a memorable performance to keep Real at bay.
In doing so, he became the first-ever Madrid senior player to be born after the turn of the millennium, with his date of birth falling on July 12, 2000.
1 - Vinícius Júnior has become the first ever player born in 2000 to play a competitive game for Real Madrid. Future. pic.twitter.com/yCdTG1fxoc— OptaJose (@OptaJose) 29 de septiembre de 2018
Previously so dominant in this fixture - on one famous occasion Madrid's fans hung a banner pleading for a "worthy derby opponent" - the Merengue have found life against their rivals a lot more uncomfortable since Diego Simeone walked through Atletico's doors as coach.
Incredibly, the last four derbies have ended in a draw, also a record.
What is more, having gone down 3-0 to Sevilla midweek Madrid's blank means they have failed to score in two consecutive Liga games for the first time since 2011 - when Jose Mourinho was in charge!
The result leaves Madrid tied at the top with Barcelona, who squandered seven points in the space of seven days, while Atletico lie two points back in fourth position.