The 33-year-old has been a free agent all summer following his departure from Paris Saint-Germain.
The striker had not been linked with Man United at all before Saturday when rumours rapidly accelerated to him agreeing a contract and then travelling to England for a medical.
Cavani was set to join the Red Devils on Monday as Ed Woodward desperately tried to bring in reinforcements.
Before transfer deadline day, the only new signing was Van De Beek from Ajax for £40million.
United’s 6-1 defeat to Jose Mourinho’s Tottenham on Saturday exposed the glaring weaknesses in Ole Gunnar Solskjaer’s squad.
United have been linked to Jadon Sancho and Ismaila Sarr – two young players who could have made an instant impact but also have room to improve – and it is thought a deal for Sarr is still alive.
Instead, they went for the veteran striker who was available for nothing. But that still doesn’t mean he comes cheap, though.
Cavani is set to become the joint fourth-highest earner at Old Trafford on £200,000-a-week, the same as Marcus Rashford.
He will even be paid more than club captain Harry Maguire.
Reports suggested he would be signed on a two-year deal, meaning United would have to hand more than £20.8million to the Uruguay star in wages.
David De Gea is still United’s top earner at an astonishing £375,000-a-week. Back-up goalkeeper Dean Henderson got a new deal in the summer, this added to Sergio Romero’s pay packet means United are spending £565,000-week on ‘keepers.
The club’s star signing from January, Bruno Fernandes, who helped propel them to a third place finish last season, earns a relatively modest £100,000-a-week.
That is the same sum given to new signing Donny van De Beek.