Manchester United have sent scouts to watch Napoli defender Kalibou Koulibaly.

The Red Devils fell 19 points behind league leaders Liverpool after falling 3-1 at Anfield on Sunday.

Manager Jose Mourinho is desperate for defensive reinforcements this January and Koulibaly is his number one target.

And according to The Sun, United have recently had people in the stands watching the Senegalese centre-back.

An Old Trafford source said: "The club had people there and it wasn't just to run the rule over Liverpool.

"They wanted to see what Koulibaly was like against English opposition after watching him a few times in Italy.

"He impressed everyone and the need for players at the back has never been more evident."

It is reported that United are willing pay £90m for the 27-year-old.