It was the 22-year-old's fourth involvement in Louis van Gaal's first team this season. He had previously featured in the league against West Ham United and Bournemouth and in the Champions League against Wolfsburg.
This was thus the first time he had been involved in a win for his side, but he has generally done well when called upon and looks ready for more regular involvement. Given United's struggles with the full-back positions this season, that readiness should be exploited.
Of course, any praise he receives for his involvement in Friday night's FA Cup fourth-round win over Derby has to be tempered by the standard of opposition. Derby are a Championship side, and thus United should be expected to deal with them comfortably.
However, the Red Devils struggled against League One opposition when it took a late penalty to scrape past Sheffield United in the third round.
They have lost to two of the sides promoted from last season's Championship in the Premier League already this season, and so there were not many Reds taking their side's victory at Pride Park for granted.
Indeed, before the game, James Robson of the Manchester Evening News wondered on Twitter whether the Rams would target United's inexperienced full-backs:
In the end Varela and Cameron Borthwick-Jackson emerged as two of United's star performers and were widely praised on social media.
Varela's numbers from the game were pretty impressive. Defensively, he made two tackles, three interceptions—the most of any United player—and two clearances. From an attacking perspective, he found a man with one of his two cross attempts and made two key passes.
The data also hints at a slightly more bursty, more erratic style than Borthwick-Jackson, something that might be a concern to Van Gaal.
He was dispossessed twice and made two unstable touches—meaning he failed to control the ball when it was passed to him. His pass-completion percentage of 79.1 was the worst of any of United's outfield players.
Borthwick-Jackson, by contrast, found a United player with 92.9 per cent of his 42 passes, made no unstable touches and was not dispossessed once. He did not make any key passes, though, nor any attempts to beat a man—Varela made two, succeeding with one of them.
Of course, as long as Varela keeps his less safe play to the opposition half, even Van Gaal may see it as an acceptable risk so long as he keeps creating chances.
The numbers, as ever, do not tell the story so much as illustrate it. And in this case, they do a particularly good job. Varela was indeed slightly more creative and risk-taking than Borthwick-Jackson—though, the latter was involved in some very nice passing interplay with Anthony Martial. Varela was both defensively and offensively effective.
The hint of flair in his game—shown with a couple of deft flicks and improvisational touches—gives United another dimension in an attack that sorely needs one.
It will be intriguing to see how he manages the step up to higher quality opposition, but he has rarely looked overawed this season. Indeed, both he and Borthwick-Jackson appear thoroughly ready for the big time.
At the moment, sending him on loan seems like it would be an error. There is surely enough talent there to make it worth incorporating him into the first team for longer periods, especially given that United are in the midst of yet another defensive injury crisis.
Bright and creative in attack, relatively solid in defence, Varela adds an X-factor to United's right-hand flank that is very welcome in what has often been a very stodgy offensive unit.
If the promise of his display against Derby was anything to go by, Varela has the potential to become an important player for the Red Devils for many years to come.
All advanced statistics per WhoScored.com.