Juve are but one of several European outfits looking to tempt Herrera away from Old Trafford, according to Italian newspaper Tuttosport, which claimed Gary Neville's Valencia are also eyeing his talents (h/t Metro's Mark Brus).
Red Devils manager Louis van Gaal has been forced to promote Herrera this season after seeing his first-team ranks hit by injuries, but the Spaniard still doesn't boast a guaranteed place in United's XI.
According to Spanish website Fichajes, Juventus sporting director Fabio Paratici and vice-president Pavel Nedved were both at Old Trafford for Saturday's 2-1 win over Swansea City (h/t Daily Star's Alex Harris).
The Bianconeri lost Andrea Pirlo and Arturo Vidal to New York City F.C. and Bayern Munich, respectively, over the summer and may be willing to offer Herrera a route out of Old Trafford should he seek it.
The 26-year-old has scored three goals and made three assists for United in 13 starts across all competitions this term, and Squawka's Greg Johnson has tipped him as one of the club's top options in midfield:
Metro's Chris Winterburn also backed Herrera to succeed with the English titans and recently hinted he could outlast under-fire boss Van Gaal:
Brus noted Herrera "could be available for between £22 million and £25 million," but with two-and-a-half years remaining on his contract, there's no rush to sanction a sale for a squad already hurt by injuries.
Yet despite those setbacks, out-of-favour Fellaini has struggled to find his chance under Van Gaal. The Belgium international emerged as a rumoured target for Milan, but Rossoneri president Adriano Galliani told Mediaset Premium there is no chance of a deal (h/t Simon Bajkowski and Samuel Luckhurst of the Manchester Evening News):
Fellaini is a player I like, I have always liked him, but there is no way he is joining Milan. I can only deny that Fellaini is close to Milan. We already have seven players in midfield. If nobody leaves in that area, we will not sign anyone. We are well stocked in all areas. It is very unlikely we will make any signings unless someone leaves.
Sport Witness translated a report from Italian daily Gazzetta dello Sport, where it was stated Milan aren't the only Serie A outfit interested in a €20 million (£14.7 million) move for the player:
Fellaini has made just eight starts for Manchester United this season and has failed to fit into Van Gaal's midfield or as a makeshift centre-forward. With Euro 2016 coming up, it makes sense the former Everton star might seek first-team football elsewhere.
However, Milan won't be the club to tempt him away, it seems, after Galliani left little to interpretation by ruling his club out of the running.