I love Marchsio but if we had signed Nainggolan I would agree with selling Marchisio for 25 million pounds, as that is a lot of money and we could have got Nainggolan for 15 million Euro. But it would be hard to find an equally good player to replace Marchisio. However, given that we have Pogba, Pirlo, Vidal and Kwadwo who can all play CM without us missing out if Marchisio is not there it makes sense to get a huge fee like 25 million pounds to put towards players we would not be able to get otherwise.. like Mata, Di Maria or Jovetic.
This deal is essentially asking if we would trade Marchisio for Mata or another superstar in that price range? It would give us a huge weapon we dont currently have while not really overly impacting out midfield given our wealth of talent there. Also we could sign a very good CM like Bradley for out bench for a moderate fee (7 or 8 million) if needed since he is now probably surplus at Roma.