I can understand the calls to keep Mattock after all he does have potential, England U21 involvement, don't want to be a selling club, must nuture our own talent etc...
But at 19 years of age IMO and pretty much two full season's of first team involvement I don't believe he has shown the kind of form to justify such a price tag. Turn it around and I would be devastated if we paid £2m for Mattock based on his career to date, and I really don't see him becoming anymore than an average premiership player at best. We are fast approaching the point where Mattock's value will start to decline if the potential does not develop into performance. Look how Joachim levelled out and we failed to capaitalise.
IMO Mattock wasn't even the best left back in our squad let alone league one, unlike Morrison and Hobbs who gre in stature and looked quality in many games, Mattock was far from being a class above and too be honest I doubt he will be a stand out player in the Championship next season.
IMO now is the time to cash in, we have a manager and scouting network who we can have confidence and trust to invest the money wisely and improve the squad.