The 6ft 5ins Belgium international has been on West Ham's radar for some time, and the French club have reportedly rebuffed several approaches over the summer.
West Ham had been waiting to get the Scamacca deal over the line, but now that appears to be nearly complete after he agreed personal terms on Monday and is set to fly to London for a medical on Tuesday.
The Irons agreed to pay an initial £30.5million with £5million in add-ons to Sassuolo for the 23-year-old Scamacca, who scored 16 goals in 36 Serie A appearances during the 2021/22 season.
West Ham can now step up the chase for Onana and are reportedly waiting for a response from Lille for their third bid of £32million. Although Sky Sports believe the French club are holding out for a fee closer to £50million.
The delay could alert other clubs to the 20-year-old's availability with Liverpool rumoured to be in the hunt.
The defensive midfielder made 42 appearances for Lille last season, scoring three goals and providing one assist.
If he joins West Ham he will no doubt support an overworked Declan Rice and Tomas Soucek in the first team squad, especially with another potential busy European campaign on the cards.
Soucek wasn't as effective in the 2021/22 campaign, and competition from Onana will only serve to improve the middle of the park.
David Moyes appears keen to surround himself with a team of giants, with Scamacca at 6ft 5ins, Soucek (6ft 4ins), Kurt Zouma (6ft 3ins), Craig Dawson (6ft 2ins) making it even harder for opposition teams to deal with West Ham's set-plays.