Let's start by prefixing this with an examination of the concept of being "overrated". Overrated does not necessarily mean bad, far from it in certain situations. Overrated simply means (thanks google): "a higher opinion of (someone or something) than is deserved."
It is in this context that Chelsea's Marcos Alonso exists as the one of the most overrated players in the Premier League. Because, although the Spaniard can be an extremely effective presence in the club's current attacking system, his defensive inefficiencies are too great to justify this.
As a Chelsea fan, I understand the incendiary nature of this opinion, especially considering Alonso's emphatic start to the season, and his place in the PFA's Team of the Year last season.