Manchester United are reportedly interested in bringing Real Madrid midfielder Casemiro to Old Trafford, while Michael Carrick is said to be a target for West Bromwich Albion, Leicester City and Aston Villa.
However, Casemiro "is not particularly keen to leave Madrid," while Paris Saint-Germain are also interested in his services.
The Brazilian has become a crucial part of the Madrid team and is able to break up attacks and shield the defence.
WhoScored.com illustrate just how effective he has been in the UEFA Champions League this season:
United may be in need of further midfield reinforcements with Marouane Fellaini out of contract at the end of the season.
Mourinho has said he is "worried" about the Belgian's contract situation, per Samuel Luckhurst at the Manchester Evening News.
Carrick could also depart in January, having become frustrated with a lack of game time this season, per Steve Bates of the Sunday Mirror.
The midfielder is yet to appear in the Premier League in this campaign and has made just one start in the Carabao Cup, according to WhoScored.com.
Carrick's situation has alerted Aston Villa, West Brom and Leicester, although the 36-year-old is not yet considering leaving United, per Bates.
The arrival of Nemanja Matic from Chelsea in the summer has seen the former England international's chances of game time further reduced.
The Serbian has proved a huge hit since joining the Red Devils and showed against Tottenham Hotspur on Saturday how crucial he is to the team, per Sky Sports Statto:
The form of Fellaini has also seen the Belgian become a key figure under Mourinho, and his recent absence through injury has seen him missed, per football writer Liam Canning:
Carrick has been at United for over a decade and enjoyed a glittering career, winning five Premier League titles, the FA Cup and the Champions League.
However, this does look to be his last season at the club, and United may well be on the lookout for a player who can provide a controlling presence in midfield.