Pjanic can fill multiple spots based on who else is available playing next to him.
Lets take some hypothetical scenarios:
1- As a CAM he will obviously be immense regardless of who else is available. He is guaranteed top quality if he will only have defensive duties. Thats the 4-3-1-2 scenario.
2- As a LCM, he can cover for Pogba when he is injured or tired. He would essentially be doing for us what he has been doing for Roma to great effect. Only there is no Dzeko upfront to waste the chances. He can thus fill this role effectively in a 4-3-1-2 (with pereyra as CAM) or a 3-5-2.
3- As a CM, he can cover for marchisio in the regista role. While he is not as solid as marchisio defensively he is not as bad at Pirlo at it. He is also superior to marchisio in his passing game and is great in possession. He can thus fill in for Marchisio in a 3-5-2 and at worst Khedira or Starurururu would have slightly more defensive duties than normal if we are faced with a tough opposition midfield. We had no problems being one the best defenses in Europe when Pirlo was our regista and Pjanic is better defensively than Pirlo. Surely, having Pirlo as regista is better offensively than anyone else but we'd still be pretty unstoppable with Pjanic now that Dybala and Pogba are there. The alternative this season has been Lemina and Hernanes and so obviously Pjanic would be a huge improvement.
4- As a RCM. Offensively he is much more potent than Khedira and Allegri can field him there and divide the creative tasks between him and Pogba going forward. Marchisio and Pogba can act more defensively than usual to maintain the balance if needed. We would be pretty unstoppable in possession with these 3 playing at once. Barca fields iniesta, Besquests and rakitic, all of which are great passers and only Besquets is defensive. We would be field Pogba for iniesta, Marchisio for besquets and Pjanic for rakitic, knowing that Pogba is much better than iniesta defensively and marchisio is better than besquets defensively too. We would be finally able to impose our standard formation against the Elites instead of playing on the counter.
The midfield combos and the duties distributions you can have with Pjanic, Pogba, Marchisio and Khedira are numerous. We would not have to be dependent on any of them in particular to be offensively creative and defensively solid. Our possession play will take a leap to the next level. We would also be much more dangerous from set pieces when he is around be it corners or free kicks (recall how many times Pirlo bailed us out with a FK). I don't think there is a better CM on the market and the cherry on top is that he keeps getting better every season and is still young. Add the Juve mentality to him and Roma's tears and this becomes a perfect transfer. He is not even expensive for what he offers.
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You mean apart from Pjanic scoring and assisting MUCH more, being much better in possession, being capable of playing CAM at a WC level, being susbtantially more experienced, and being about as solid defensively.
This is when no one really watched him enough