Two possibilities if he starts in the CL, either he completely fails under the pressure or he thrives big time. If it comes down to it, I think he'll do well. He seems to be a pretty confident kid, he did very well when he was thrown into a starting role in the first game of the season, and seeing as we are playing a French side this could also give him a boost to play harder. Regardless, I don't think he will get a start, Pereyra is going to be CAM and Sturaro is going to slot in at mid.
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IMO whoever comes into Marchisios role isn't our biggest concern though, our biggest concern is whether Pirlo and Vidal can step up and play well. They've both been struggling heavily at times, but Pogba and Marchisio's form has masked that all season, with them gone, Pirlo and Vidal need to step up big time and play like they know how to.