Sime Vrsaljko
After arriving at Atletico Madrid for a fee of £14.5 million from Sassuolo in 2016, Sime Vrsaljko has steadily improved and become an important part of Atleti's squad. He arrived at the club as a stereotypical attacking full back, assisting the attackers by making constant overlapping runs. Though strong in the tackle, Vrsaljko had the tendency to jump in a bit early. Although, as you'd expect, two years under the tutelage of Diego Simeone have ironed out most of his defensive flaws.
With Juanfran getting on in age, Vrsaljko has a very bright chance of becoming Atleti's undisputed first choice right back next season. However, the Croatian is not considered indispensable and if United come up with a bid in excess of £45M, the top brass at the Vicente Calderon might consider their offer.
Recently, Simeone's side have been linked to Real Sociedad's promising full-back Alvaro Odriozola and their reported interest in the young Spaniard might help Mourinho's cause in prising Vrsaljko away from Madrid.