ESPN FC is reporting that the Red Devils boss has “reignited” his interest in signing the midfielder this month.
The same report claims that Mourinho’s recent trip to Belgium during the international break was focused on scouting the 29-year-old midfielder.
It is claimed in the same article that Mourinho is a long-term admirer of Witsel and that Manchester United had considered a move to sign him last summer.
Witsel has scored one goal and made one assist in 12 Bundesliga appearances for Borussia Dortmund so far this season.
The article goes on to claim that Mourinho is “determined” to improve his midfield options at Manchester United as he looks for his team to bounce back from a slow start to the season.
The same story says that Paul Pogba’s long-term future at Old Trafford remains “uncertain”.
The ESPN FC report also claims that Dortmund may be reluctant to let Witsel go in January, but a significant bid could tempt them into selling next summer.
The Red Devils currently find themselves in seventh place in the Premier League table and 14 points behind leaders Manchester City.
They will return to Premier League action on Saturday when they travel to Southampton in the late kick-off.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news