You need to think about what the players want too. Are the players happy to give up their chances of playing at Wembley to improve the chances of Sheffield United having more money to be able to sign better players? If the players are a bit teed off by the team changes, it won't make them consciously want to lose, but it might not help their mood for the league battle.
There are people on the Burnley board want to make 6 or 7 changes for the Fleetwood game, because "the league is more important". You're 7 points clear, we're further than that - stop mithering, play your first team, go and beat them. Being afraid of losing is never a reason for not trying.