Soler has quickly established a reputation as one of La Liga's most promising young players since making his senior Valencia debut in December 2016 and last month it was suggested that long-term suitors United were attempting to seal a £30m ($39.4m) deal for next summer.
The Spanish Under-21 international's current contract, which still has three-and-a-half years left to run after being extended in May, actually contains a €80m (£71.9m) release clause.
However, it has been claimed that rejuvenated Los Che, if they do not qualify for the Champions League or receive a cash injection from owner Peter Lim, may have to sell for a cut-price fee as they look to generate €45m to help balance their books after years of underachievement.
Free-scoring Valencia, 12th-place finishers in 2016-17 who won the last of their six domestic titles in 2003-04, are currently second after winning eight and losing none of their first 11 La Liga matches under the tutelage of former Villarreal boss Marcelino Garcia Toral, who became the club's sixth manager in just two years after replacing interim coach Voro in May.
The top four teams in the Spanish top-flight will automatically qualify for the group stages of the 2018-19 edition of Europe's top club competition after a recent Uefa rule change.
According to the latest reports from The Independent, Soler currently tops Mourinho's wishlist for the next window after the Portuguese became increasingly worried at how the absence of Paul Pogba had adversely affected his team. The influential France international has not played a single minute of football since limping out of a 3-0 victory over FC Basel in September with a hamstring injury.
While United have not exactly cratered without Pogba, the lack of creativity has been painfully apparent over recent weeks - especially in big matches away from home - and they now sit eight points adrift of rivals Manchester City in second place in the Premier League.
The Independent states that United will be far tighter with the purse strings in January than they were during the summer and that as a result Mourinho, while still chasing a winger amid renewed links to RB Leipzig's Emil Forsberg and a previous failure to sign Ivan Perisic of Inter Milan, is set to concentrate on strengthening his midfield.
The same publication mentions £40m as a potential price tag for Soler and adds that the Red Devils have also considered a player-plus-cash deal involving Andreas Pereira, who has been impressing with Valencia of late and recently stated his intention to remain at the Mestalla beyond the end of his season-long loan deal.
Further midfield reinforcements are likely to be required for United amid speculation that they could risk losing Marouane Fellaini for nothing next summer. Contract talks with the oft-maligned Belgian international - who turned down the offer of an extension in September - remain deadlocked and it is looking increasingly possible that he will depart, despite being highly-regarded by Mourinho.