Manchester United have reportedly decided to up their interest in Napoli defender Kalidou Koulibaly.

Manager Jose Mourinho is eager to strengthen his backline in January having missed out on a number of defensive targets during the summer, and Koulibaly is said to feature high on his shortlist.

According to The Sun, United opted to keep a watchful eye on the centre-back when he featured against Liverpool in the Champions League on Tuesday night.

Scouts were already scheduled to be present due to United's Premier League fixture with Liverpool five days later, but the report claims that the performance of Koulibaly was also being assessed.

United are allegedly willing to pay in the region of £90m to secure the signing of the 27-year-old, who only recently penned a new long-term contract in Naples.