To be fair we seem to always lose our top top (non British) players to Madrid/Barca, with the exception of Beckham.
Off the top of my head we've lost:
Ronaldo - Portuguese (balon d'or winner, with us, Talismanic #7, best player in the world at the time)
Ruud - Dutch (best goalscorer in europe. Absolutely lethal finisher that any team in the world would want ala Haaland)
Becks - English (commercial brand like no other. Every kid in the country wanted to be beckham. Talismanic #7)
(Almost) De Gea - Spanish (only world class player in our team at the time. probably best keeper in the world at the time too - if not for a dodgy fax machine he'd be theirs)
Honorary mentions go to Pique (won the CL with us and wanted to go back to barca), Heinze (no idea what his deal was) and Chicharito
(ok not amazing but tell me you wouldn't have him in his prime right now).
Trend is - a huge player comes through United's ranks - usually foreign. To them, Madrid are fecking massive and who they've dreamed of playing for since they were 6 years old.
Madrid come in with a bid - player wants out and we have to sell.
Can't really think of any modern world class players we've lost to them in the last 10 years (hint - we haven't had any)