The former Red Devils centre-back is having an impressive season in the Premier League for newcomers Luton Town.
Mengi was a highly-rated prospect at Man United as he rose through the youth ranks, but he only played 35 minutes at first-team level.
And after multiple loan spells, his 14-year association with his boyhood club ended in August 2023, as manager Erik ten Hag moved him on.
The 21-year-old joined Luton following their first-ever promotion to the top flight.
Since then, his stock has massively risen, and Mengi attracted plenty of interest in January with Fulham and Crystal Palace reportedly looking at him.
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A move didn't happen last month, however, more impressive performances since have led to clubs in Europe catching wind.
Bundesliga leaders Bayer Leverkusen are now keeping tabs on the Luton defender with scouts being sent to watch him last weekend, according to The Mirror.
Leverkusen are reported to have tasked their chief UK scout Matthew Green to watch Mengi following talks with the Hatters' chief executive Gary Sweet.
He unfortunately didn't feature in the game against Sheffield United due to an injury, so it is likely the Germans will have to visit again.
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The interest in Mengi will not have surprised Man United, as they had seemingly already planned for a situation like this.
They included a substantial sell-on clause in the deal with Luton, meaning they will receive a big cut of the fee should he join Leverkusen.
United could see a future deal happening and their decision to install a clause in his contract seems like it may pay off.
Hatters boss Rob Edwards won't be keen to let Mengi go though, despite also being aware of the fact interest from bigger clubs would be coming.
Speaking about rumours of his exit in January, Edwards said: "We knew this could happen longer term.
"But he's done great so you can understand it, a 21-year-old, performing so well against the best players in the world. I understand it, but he stays with us.
"It's something that's really important to us that we keep this group together. Gary [Sweet] has reiterated that to me as well, unless we want someone to, no-one leaves."
Luton host Man United this afternoon, with Mengi and fellow former Red Devil Tahith Chong looking to get one back old club.