You quantify it by watching games and looking at the player who's the most influential in possession and out of possession. He's the heartbeat of his team. Sets the tempo, is the proper playmaker of the team, recognises what spaces to infiltrate at the right time, makes the best decisions at the right time etc... Pogba doesn't do much of that yet. At Juve, Pirlo controls and sets the pace of the game for them. A controller is pretty interchangeable with playmaker. Think Xavi, Pirlo etc. I'm not advocating that we should buy Thiago btw, literally just stating that he's a better player because he sees the game a lot better than Pogba does atm. Pogba is still learning how to do that but it's just natural for Thiago.