Erik ten Hag has named De Jong his top target for the summer transfer window, and while United has a €85 million (£71.7m) deal with Barcelona in place, the Netherlands international is wary of a move to United.
De Jong has also made it clear that his preference is to remain at Barca, while the Spanish club still owes him around €17m (£14m) after he agreed to defer part of his salary to help their financial situation.
United also face competition from Chelsea as they have also held talks about the prospect of signing De Jong this summer and it’s understood that the 25-year-old would prefer to join a club that plays in the Champions League.
According to Goal, United is exploring alternatives and Ten Hag views Ruiz as ‘an ideal alternative to De Jong.
The report also claims that Leicester City midfielder Youri Tielemans and Wolves’ Ruben Neves are two other names on United’s shortlist.
Ruiz was one of Napoli’s standout performers last season as he helped Luciano Spalletti’s side finish third in Serie A.
The Spain international has just one year remaining on his contract with Napoli and he has already rejected their offer of a new contract.
Earlier this summer, Napoli had been holding out for a €30m (£25.3m) fee for the 25-year-old.
Speaking on Friday, Ten Hag remained adamant that United will not bring in a new midfielder for the sake of it if they miss out on De Jong.
‘I have a squad and I’m happy with the current squad, we make good progress, I’m happy with the signings until now,’ said Ten Hag.
‘You don’t need any player, you need the right player, that’s what we’re working for.
‘If it’s not the right player, no [we won’t sign them], we need the right players.’