There are 90 minutes in a game so a player completing 10 league games in a season will have played 900 minutes.
Leigh hasn't managed that for the last 3 seasons so he isn't even a 10 game a season player never mind a 20 goal a season player.
Someone posted on here that he is on £19k a week so let's say that's true. That means he still has another £1 million to pick up in wages from Celtic before his contract ends. Therefore at the end of this season Celtic would probably have to pay him £700k to go away and that would allow him to go to Aberdeen or Hibs for £4k a week and make the same money.
However, Celtic is his home so he probably wants to stay there as long as he can, tieing scarves to goal posts when he is fit enough and available to do so.