Well when you have 2 (Stu/Marchisio) rumored to be leaving and since we had a weak midfield last year...2 leaving, 1 coming so far...it make sense to reinforce. Don't think matuidi will be a starter by October.
Mids: Pjanic, Can, Kovacic/Golovin, Sami, Matuidi
Out: Stu/Marchisio
6 seems like a healthy mix for 3 man midfield.
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Looking at that, we still don't really have a midfielder that can surge the ball forward quickly and effectively, which is why I mentioned Golovin/Kovacic.