While the Belgium international will now line up for his fellow countryman's arch-rivals, Kompany has claimed that he is "happy" for the 25-year-old.
Kompany wrote on Twitter: "Congratulations Marouane Fellaini! Any countryman doing well, I'm happy for. So now we will be neighbours. Careful I can be quite noisy!"
United managed to clinch Fellaini's signature in the closing stages of deadline day for £27m, which is £4m more than the midfielder's release clause, which expired in August.