Ronaldo has a year left on his contract with Juventus, but that has not stopped intermediaries from assessing whether there is appetite elsewhere for the 36-year-old.
Ronaldo and his camp have yet to make a decision on his future, and sources have told ESPN it is unlikely to be finalised until after the European Championship. Portugal begin their campaign against Hungary in Budapest on June 15.
Ronaldo has already spoken to new Juventus manager Massimiliano Allegri after he replaced Andrea Pirlo as boss, and more talks are planned before the Serie A side return for preseason training.
Sources have told ESPN that a move away from Turin has not been ruled out, but Ronaldo's wage demands and Juventus' asking price for his rights may prove to be stumbling blocks to any potential exit while clubs are still recovering from the financial impact of the coronavirus pandemic.
Ronaldo scored 36 goals in 44 games last season, and while Juventus won the Coppa Italia, they were knocked out of the Champions League by Porto in the round of 16 and had to settle for a fourth-place finish in the league, 13 points behind champions Inter Milan.
Sources have told ESPN that United have not ruled out signing a striker this summer despite agreeing to a one-year extension with Edinson Cavani.
There remains interest in Harry Kane, but there are concerns about the financial outlay it would take to secure his exit from Tottenham because the 27-year-old still has three years left on his contract.
Manager Ole Gunnar Solskjaer will be given funds to strengthen his squad but has been told the club are working on a plan of bringing in three new arrivals before the end of the transfer window on Aug. 31.