Quick question for everybody: at what fitness level do you refuse to play somebody, and at what level will you usually keep a player out of a game?

I ask this because generally I won't play anyone under 95% fitness unless it's a really important match, but I'm worried that I'm using my reserves way too much. For example, when I played in Ukraine, which just has a 30 game season and a long winter break, it would be pretty typical for some of my first team players to finish the season with 16-18 league games played because they would be playing frequently midweek.

anything below 90% is a no go, if its an important match a second half stubstitute

normally I don't play anybody under 93%, however in important matches 90% is the limit. anything lower gets to be a supersub.

depends. proly under 90%. Unless it's a very special game situation. Good rotation/well placed/timed subs is the key.

No one under 90% unless I have to, and it depends on the fitness level. If it says they're tired, even if they have 94% it's a no-go. I think I read somewhere that 15-20 games a season is enough to keep players developping at a rapid level, and I usually have good reserves so I don't have a problem with using a rotation policy.

Fittest players play, barrinf a few exceptions, assuming the best players are fit enough.

Pretty much same as everyone 90% and below is a no, just slightly above 90, depends on the situation.

under 90 for me, if my squad is deep Im looking at nothing lower than 95

95% is the bare minimum for me, it's a matter of trusting your subs.
Obviously I try not to weaken the team in all the sectors, one or two changes on each due to condition, max.
Of course it's also important to keep your main starters playing almost every match so that they remain fit and keep a good run of form going.
Mixing these factors usually does it for me, I hardly come across a depleted squad selection...

Still, it isn't such an easy thing to do when you have 4 or 5 injured players, as you do in FM... Or is it better with the 9.0.3?

It's better, but it can still happen. At any given time I tend to have at least one guy injured. Also, in-match injuries are more common, and training injuries are less so in my experience. More realistic, but it means your stars are more likely to be injured.