Nkunku has 2 goals and 1 assist in 390 minutes of PL football, 1 goal in 1 game in FA cup, 5 goals and 3 assists in 380 minutes of ECL, 3 goals in 160 minutes of League Cup.
Just because he has no starting place with Palmer starting PL games, doesn't mean he hasn't performed when he actually gets to play. His numbers are incredible.
To call it a "massive disappointment" is disingenuous to put it mildly. In fact he has scored vastly more goals this season, 13 in all competitions, than any of our players despite not being a starter.
What would be best for him would be a move to Man United as Rashford replacement. He is a top player that is wasted at Chelsea and who would be an ideal fit for us.