The reason I think we will sell him this season if he doesn't improve is because our midfield is starting to look rather weak.
I disagree. In theory, having two of our important central midfielders pushing the bad side of 35 is not great, but in practice their performances recently have improved from last season. This is an era where all our players have been emplored by the fans to "step up" after the loss of a special player, and two of the players that are stepping it up the most happen to be two of our longest employees. As for them both retiring within 2 years, I think this is taken as a given now.
Park is just Park, you get what you pay for. There's no point in critiscising Hargreaves when he can't even respond, when he is fully fit I think he could be an extremely important part of the team towards the latter stages of this season, especially in Europe.
Carrick has been off-form recently and not playing anywhere near as well as we know he can (and has done) previously, so it's just a matter of time for him to regain that confidence he had and sort himself out.
Fletcher has come on leaps and bounds in the last few seasons, he certainly has the makings of a midfield general. He's gone from being a solid reliable squad player to an extremely talented first teamer, and any midfield containing him can't go far wrong.
Is our midfield looking weak? Each to their own if you think so. Of course there is always room for improvement, but any team in the world would kill for the midfielders we have. This really is a championship winning midfield.