I play rotation all the time, and in my experience your player won't be unhappy if you're on winning runs and they get roughly 20-30% of all games available (consistently though, don't play them a handful of games and then left them out in all competition in the next month

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First rule obviously that your backup should be good enough to be able to take on league home game against bottom place team... if your backup can't even handle that then you shouldn't want to keep him anyway
I have 2 teams basically, I play 4-4-2, Team A take on the top of the table team's Away and sometime home games, Team B take on bottom half teams home and at occassion away games. Look at your schedule and prioritize, see where your next break (where you have 5 days+ without a game) comes and then decide which team you should take on each game.
The players that I designate to be backups HAVE to be versatile, they need to be able to play in at least 2 positions so I can have options on where to play them.
Coming on as substitutes helps to an extent, so give them at least 15 minutes of sub time when you can
but in the end it's about priority, give your backup player games on matches that you should be winning and let your first team take on the tough ones
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