Number one - remind yourself that you are very lucky to have them (especially as they rarely, if ever, moan)! This will communicate itself to them and they may actually contribute more.
Two - don't expect the impossible - if they seldom get to bat it's not their fault that they couldn't save the game for you.
Three - a piece of advice I was given many years ago about cricket, and it proved to be true. 'Every dog has its day.' As an example, one of these fill in players, who had never ever reached double figures with the bat, went in against a seemingly unplayable attack and cracked 90-odd not out. OK next week it was back to normal, but on that day it was back to point number 1.