According to The Sun's Andrew Dillon, the midfielder believes he no longer features in the plans of Blues manager Antonio Conte, but he's on Mourinho's wish list of targets.
Matic won the title under Mourinho in 2015, and he appears willing to work with him again despite appearing to fall out of favour with the Special One. The midfielder was even hauled off 28 minutes after being brought on against Southampton in October that year as the Portuguese's Stamford Bridge reign fell apart.
Matic formed a robust partnership with N'Golo Kante in Chelsea's midfield under Conte last season as the west London club won the Premier League title, but he could be viewed as the more expendable of the pair should the Blues bring in any midfield reinforcements this summer.
United sold Morgan Schneiderlin and Bastian Schweinsteiger in January, while Michael Carrick will be 36 in July, so reinforcements in the centre are needed. A holding player could give Paul Pogba even more licence to get forward, and Matic may fit the bill.
According to Squawka, Matic won 37 tackles in the Premier League last season, as well as making 55 interceptions and 49 clearances.
He showed his technical quality, too, with seven assists and 27 chances created in total, and he also maintained a passing accuracy of 88 per cent.
Squawka's Muhammad Butt does not believe the Serb would be a strong choice, however:
James Robson of the Manchester Evening News also believes there are better candidates out there:
There are indeed better, younger and more energetic players Mourinho could bring in than the 28-year-old, and Fabinho would be an outstanding option, as Robson noted.
Nevertheless, Matic would make for a significant upgrade on the likes of Marouane Fellaini, and he's played an important role in two title wins for Chelsea in the last three years. The Serb could be attainable if the Blues strengthen their midfield options this summer.
Matic may not be a name to excite many of the Old Trafford faithful, and there are better—albeit likely more expensive—players United could target instead, but he could still prove to be a strong buy.