Ah... But therein lies the heart of the problem, and it's a problem that a lot of managers fail at.
When a team is winning/losing, conventional wisdom says to sub on either a defensive player to protect a lead or an offensive player to apply more pressure. That ignores possibility number 3 which is to continue what has been working.
Bringing Mix on if we're trailing wouldn't necessarily be an offensive spark, but he could stabilize the midfield/regain control to be another guy that makes the crucial pass.
Bringing him on if we're winning also stabilizes the midfield and doesn't allow for a bunker mentality. Too many defensive minded players turn the ball over after creating a defensive stop because they aren't skilled enough to hold the ball and initiated counter attacks.
Save for very particular situations, I very much think Mix could and should be the first midfielder off the bench.