I thought Yaya was the best midfielder in England last season, he was City's best player en route to them winning the EPL, IMO he had a slightly better season than Vidal, who in comparison to other players in Juve, you could argue Pirlo and Tevez were more instrumental, its also arguable who was more impressive between Pogba and Vidal, even though this might be me biased to the Frenchman
Though its tough to compare Yaya and Vidal, while both are box to box midfielders, I feel they offer different qualities. If you want your midfielder to defend, and be a good tackler, no doubt Vidal is better in that department, Vidal also has more stamina and energy. Yaya on the other hand is more technically sound, can score more goals(even though Vidal scores a lot of goals for a midfielder too) is stronger physically and is a much better dribbler. Yaya on his day is a slightly better player in my opinion, its pretty close though and you can make a compelling argument for either one.
Vidal is probably more all rounded than most midfielders out there, I'll give you that, I don't find any glaring weaknesses in his game to be honest.
I have no problem with people claiming he's the best box to box midfielder, its a perfectly valid claim, its definitely not a fact though
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not sure if serious.