And, for the Red Devils, their former youngster's continued success is not only a kick in the teeth, but it also leaves egg on the faces of their transfer team whose big-money deals have not worked out as planned.
Elanga was one of a host of players shipped out by Man United in the summer as Erik ten Hag revamped his team.
The 21-year-old Swedish winger was previously regarded as one of the club's most promising young talents, but found himself on the fringes under the Dutch boss who brought in Antony from Ajax in his place in August 2022.
But that decision appears to have backfired, with Elanga thriving since his £15million switch to the City Ground, while Antony has struggled to prove good value for the £86m United spent to sign him.
But it's not just the Brazilian ace who Elanga has outshone this season.
Read more on Manchester United
Why are you questioning me?' - Ferdinand in heated clash with Jenas over Man United 'bias'
Man United and Ten Hag told to take responsibility for struggling stars after 'panic buys'
The Forest star set up Morgan Gibbs-White for the winning goal in Saturday's 2-1 victory, which put him into double figures in total goal contributions this season with four goals and six assists.
That's more than United's entire attacking strike force combined this term, with Marcus Rashford, Anthony Martial, Rasmus Hojlund, Anthony and Jadon Sancho - currently exiled by Ten Hag - managing just nine direct goal contributions between them.
In fact, Elanga's record puts him alongside Real Madrid star Jude Bellingham in the top-five most prolific players aged 21 or younger in Europe's big five leagues so far this season.
It's clear Elanga is relishing the chance to prove his old club wrong. You'd be stretched to find a better motivator in the world of football than trying to beat your former club, especially if you felt undervalued and unappreciated.
Most read in Football
Why are you questioning me?' - Ferdinand in heated clash with Jenas over Man United 'bias'
Premier League done deals: Every completed transfer in 2024 January window
Salah to Toney, De Gea to Messi - the biggest deals in a potentially mega January window
Man United and Ten Hag told to take responsibility for struggling stars after 'panic buys'
The 21-year-old even pointed to the Tricky Trees' badge when he was substituted late in the game, and looked visibly emotional after the full-time whistle as he hugged new manager Nuno Espirito Santo.
For good reason - not only was it his first win over his former club, it was Nottingham Forest's first victory over Manchester United in 29 years.
Simon Jordan slams Erik ten Hag's comments about Manchester United's transfers and calls Antony £90m worth of crap
And it appears he enjoyed it, very much.
He told NBC Sports after the game: "Last season wasn't easy for me because I'd play for five or ten minutes and then I wouldn't play for ten games, I'd be in and out a lot.
"So this game was kinda personal for me because I wanted to prove that I'm capable of playing at a high level.
"I'm just grateful to Nottingham Forest for bringing me in, especially Steve Cooper who has obviously left and now Nuno and his staff have come in and done a tremendous job so far.
"Before the game I tried to put my emotions aside and just stay as professional as I could… and the assist to Morgan was special to me because that puts me now on double figures.
"To think that last year I didn't score, and now I'm on four goals and six assists, I just want to keep pushing and keep working hard."
Man United will now fear that the more he works hard, the more silly they'll look for getting rid of him.