If you're playing as Atletico Madrid, then I wonder why people are suggesting players like Pogba and Vidal. Those guys are too pricey for a selling club like AM.
I would suggest Lucas Romero. No matter what team he plays for in the game, he always develops really well. He's literally a poor man's Yaya Toure. Incredible physical stats, while everything else is quite balanced.
A solid and cheap midfielder I always sign (if possible) no matter what club I start with.