In my current save, I think this season is one of very few in my whole fm journey where I have managed to keep every single player happy, normally I don'tcare as long as they play well haha.
Things I have done to manage that is:-(Not all of this is about squad roatition I get, but it all ties in to keep players happy and fresh. When rotating players poorly you can annoy them but if they are already very happy they take being dropped/rested in their stride, so bear with me lol)
Firstly (I had a created club with no players so my route will be different to othes but after the squad building part the rest should apply to yours), with squad building I brought in 2 starting 11's and about 6 players or so on top of that who between them had the attributes to cover multiple areas of the pitch. No need for huge squads if you want to keep them all happy. Luckily for me with having to build a squad from scratch I could get in players on contracts that didn't demand they play every game, though I do have a few that wanted star player status
I set the training intensitys so that the first 2 options were no training/rest, whatever it is(I don't have the game open so I am going by memory lol) and the last 2 as double intensity, low and medium, or whatever it is, inbetween them
I keep an eye on training to make sure every week I get reports saying all players had great sessions. If players get poor training ratings I criticise them and if they get ratings of over 8.5ish I praise them, both improve morale a bt.
If players who haven't played in a while get a high training rating and the next game or 2 are games they canplay in without the team suffering or not a huge game they shouldn't play in, then I give them a start. Same goes for players who get a run in the team, if their performances start dropping, then I will give someone else a chance. In my team you get chances to show your worth to the team or you get replaced.
Unless I really have to, like because of injuries/suspensions/poor form, then I never start players with either 92% or less fitness levels or 90% match sharpness, if both are lower then they get benched or dropped. Though with match sharpness with my often rotating or making them available for the u21s/u18s, it's not usually an issue too much.
With my 2 keepers, neither have a contract that states they are number 1 so it's not too bad but I still roatate them. I have one I consider to be my number 1, just not told him that lol, he starts the majority of games and finals. My "backup" is my cup keeper as well as I play him in league games against some teams I know I can control and limit there shooting.
In team talks, I rarely go too hard on them unless they were so shocking they deserve the boot kicked at them. I often praise players individually in the team talks if they have got over an 8.5. I paise them after games if they get potm etc.Not had it yet but if the press ask anything about them in interviews I would say whatever option would be the "nicest"one lol
I try and make sure every squad memeber has had at least 10 starts and multiple sub appearances by the end of the season. Good for their morale and keeping everyone fresh. This season I haven't really had that many injuries, apart from 1 young player who gets injured more than IRL martial and bailly combined lol, but he will be gone soon.
Off the top of my head that pretty much covers it. To manage squad rotaion correctly after fitness/sharpness levels, keeping players happy is extemely important so they don't start complaining, that's where things can go bad quickly if you get it wrong.