Assombalonga joined Middlesbrough in the summer of 2017 when he moved from Forest for a transfer fee reported by BBC Sport to be worth £15 million.
The 25-year-old striker has a superb goalscoring record, and he did continue his impressive form at his new club, scoring 15 goals in 34 Championship starts.
The Northern Echo reports that Tony Pulis’s side will not sell him on the cheap this summer.
Assombalonga is a fantastic striker who has a proven Championship goalscoring record, and he would be a good signing for Forest or City.
He scored 14 goals in 20 Championship starts for Forest in 2016-17 and 15 goals in 27 league starts for the Reds in 2014-15. He was a fan favourite at the City ground, but it remains to be seen whether Forest can bring their former man back at the club.
Forest have been very active in the summer transfer window, and they have signed Lewis Grabban on Friday. He has joined for an undisclosed fee from Bournemouth, believed to be in the region of £6 million, and it remains to be seen whether Aitor Karanka will still push for a move for Assombalonga.