I agree Felix would be a particularly bad deal. However, I think the reason Garnacho is being sold is entirely to do with FFP. I am not an expert on FFP, but if Utd sold Garnacho for £50m, that would give £50m on the books instantly.
Then if Nkunku came in for £60m and Dorgu for £35m over 5 seasons each, their transfer book value would be £12m and £7m respectively - I won't go into wages as too much guess work. So, that gives an extra £31m on the books in transfer fees. Isn't that the kind of thing exactly what Chelsea have been doing since the new owners have come in and Utd need to do to get some players in?