Fair enough regarding the 4-3-3-argument. Diamanti would obviously be best suited for that tactic, and we don't seem to use it anymore. But we did use it a lot more when Pepe was around, though, and I think we would utilize it more if Diamanti was to be signed.
I wouldn't want us to pay 10 million in cash for Diamanti, but 4-5 millions + a fringe player (depending on who, ofc) wouldn't be a bad deal.
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Signing him would give us more options regarding formations. I believe we would see the re-introduction of the 4-3-3 in certain games