Manchester United have not even shown serious interest in Osimhen and even till now they're not really focused on him. Their main focus for now and Gyokeres. They believe the influence of Amorim can convince him to join Manchester United. I don't see that happening if they don't make it to the Champions League.
But what do you mean Chelsea has not made such a big signing since Abramovic left? The same Chelsea that signed Enzo for 100 million or the Chelsea that signed Caicedo for 100 million? Which of them? The Chelsea that signed Mudryk for 89 million? All Chelsea has been doing since Abramovic left is make big money signings on players that are not worth half of that.
Do you follow FabrizioRomano on YouTube? Last two weeks he has said something relating/linking Osimhen with Manchester United. The move for this pal to Manchester United can only happen if they make it to the Champions League Competition next season. At the moment, no direction communications has been made between Napoli and Victor Osimhen.
Considering the age for Gyokeres, I don't see Manchester United getting him. Haven't Chelsea spent $1B+ in the last two seasons already? I'm no blue but, in the last 20 years, I know Chelsea to be the most decorated premier league side in Europe. They did better under Roman Abrahimovic.