I think make muzz a "last 20min striker". As that sub he isn't really expected to achieve too much because that heavily depends on the disposition of the game, versus the pressure of carrying the front line for 90min. It might allow him to get outside of his head a bit and have a go. Also lets him run at tired defenders. And if he comes on if we're 2 nil down, nobody really expects a goal at that point. So a goal becomes a bonus, rather than an expectation (i know I'm understating the role of a sub) Though i don't think it's just confidence, but hope I'm wrong.