You can see in the new thread a statistical (but incomplete) explanation of what exactly the arrows do to the players stats.
I usually play players based on form arrows, preferring those on better arrows. When a player's green stamina bar goes down, he is rested if I have enough cover. Fatigue bar works like this too. If you let the fatigue bar rise too much, there is greater chance of them getting injured so rest them a bit and the fatigue should reduce.
Buying new players in ML mode reduces their teamwork stats and this is built back up by playing lots of matches. It's better to bring in a few new players into your team rather than a whole bunch. I bought 12 players and played 10 of them in the first league match of the new season. I lost the first 2 matches and they played quite badly. After about 12 games, they're much better.