Needing just a goal to seal their place in the Barclays Premier League, debutant Bonham, 19, was at fault to allow Ross McCormack's ambitious chip to end up in the back of the net, ending the Hornets' automatic promotion hopes.
Bonham was only given his chance due to injuries to both Manuel Almunia and Jonathan Bond, but after a nightmare first appearance, Loach is confident the youngster can bounce back.
Bonham earlier tweeted: "Massively disappointed but can only move onwards upwards! I believe in myself, my ability and have the best people around me to go forward."
Loach, now with Ipswich, replied: "Mate ur a top lad and gk, u will b fine. Loved working with u. Real talent. Keep your head up."
Bond is definitely ruled out of Thursday's play-off semi-final first leg against Leicester after a collision with Dominic Poleon resulted in a suspected broken nose but Almunia, who sustained a hamstring injury in the warm-up, is expected to start.