Think the booing started while he was still on the pitch. At first I thought it might have been bcos he was being subbed rather than other players. As I could see he knew he was being subbed as he started to take his wrist bands off and made his way towards middle of pitch.He didn't help himself by not joining his team mates on the bench...... Instead Ndombele just huffed himself straight down the tunnel into the changing room.
That's what got most fans booing.