Fairly obvious reason really - when he got injured he was an out-and-out winger and the talisman for a team playing 4-4-2. He’s come back and had to play in central midfield for a team that doesn’t use wingers and where two teammates have realistically overtaken him in the ‘star man’ stakes during his year out.
Which isn’t to say he’s been bad at all, probably 7/10 and a decent performer all season. Just feels worse because he was so good before.
Anyway, on selling him, there’s two conversations to have really. Selling him at all in January feels entirely plausible for a variety of reasons. Selling him to Chesterfield (or Wrexham/Stockport etc) is a whole different matter.