The 19-year-old was all set to link up with the League One side after Black Cats boss Steve Bruce had given him permission to leave the Stadium of Light to gain some more first-team football this season following a seven-game stint at Charlton last term. However Abbott has revealed that Waghorn will be joining the Foxes instead.
"I am massively disappointed," Abbott told the News and Star.
"It was a target we have been keeping warm all summer but now he is going to a Championship club.
"We got a firm 'go-ahead' from his manager and I spoke to the player yesterday. He was almost apologetic when he said he had to take the opportunity to play in a higher division than ourselves.
"We wish him all the best and thank his club for giving us the opportunity to speak to him. But I am extremely frustrated and more disappointed than the fans can possibly be about this."