The 22-year-old was David Moyes’ first signing after he joined Man United from Penarol in 2013 but he spent the season on loan at Real Madrid Castilla.
Varela has been handed an opportunity back at United in recent weeks and he impressed in the Champions League defeat against Wolfsburg earlier this month.
United are unwilling to let Varela leave in the January transfer window as they are in the midst of an injury crisis but Napoli will test their resolve next summer.
Varela has been thrust in at the deep end recently and has performed admirably but it is clear that he is not of the required level if United are to consistently challenge for major trophies.
While a good back-up option, it is hard to see Varela staying at the club beyond next summer.