Condition recovery between matches is a function of Natural Fitness, Training, Injury, and Jadedness (may be others, I like to think Determination, Ambition, and Professionalism play a role as well given how intuitive the attributes are designed, but I couldn't answer that). There is a natural rate at which condition recovers (tests from the FMRTE forums on FM11 pegged it around 5%/per day) for every player, however the individual stats for each player then determine his real condition recovery.
As WJ said, Natural Fitness increases Condition recovery in between matches. Higher the value, the greater the effect.
Training has a negative effect, the more intense a player schedule the lower his Condition recovery. The game also seems to put more weight on the physical attribute training more for the effect of the negative multiplier. The more a player works on physical attributes, the lower the rate he will recover Condition in between matches.
While injured Condition recovery goes negative. There is a faux lag that Injury creates: Injury dumps Match Sharpness and Condition, low Sharpness dumps Condition during a game leading to a greater Condition gap to recover. This is also a place where I think other factors contribute, namely your physio. The Motivation attribute of the physio has an effect on injury layoffs, which would decrease the 'lag' in recovery caused by injuries.
Jadedness is the tricky one, being hidden and a function of other things. You can call it mental tiredness and the only indication of it building up in game is on the fitness view of the players with descriptions "Tired." There's "Tired" and "Tired after his last match." The first hints to jadedness level, the second is just the physical tiredness after a game. Natural Fitness plays a role, the higher it is the slower the rate of Jadedness. How physically demanding his tactical role is during matches in relation to his Stamina and Natural Fitness: Defensive Forwards accrue Jadedness faster than a Poacher; aggressive counterpressing forward accrues it faster than a standoff let them go right by you forward. This is why fixture congestion leads to Jadedness. Travel time has an effect: John Brown playing league games in a small country like Andorra will not get jaded as quickly as the same John Brown playing league games in the United States. As a player gets jaded, his Condition recovery rate goes down, amongst other things. Young players get jaded quicker, old players also get jaded quicker. Players set in their ways ~22,23 to ~32,33 don't have an age multiplier for Jadedness.
Training intensity has an impact on how quickly Jadedness decreases. Training does not increase Jadedness, at worst it seems to keep it constant; nor does playing in non-competitive reserve games. (In my save, Antonio Romero plays for my B team (Playing as TSV 1860) and the league the B team is in isn't loaded. The B team is set to play 6 friendly fixtures in 4 weeks all throughout the season. Romero has played in practically every reserve fixture, yet his Jadedness is being clocked at -500.) Lower training intensities help decrease Jadedness and time off from training (rest days) also decrease Jadedness. Having a player be rotated out of playing a game will decrease his Jadedness, but not as quickly as time off from training.
These two screenshots are just 16 days (3 matches apart):
View attachment 170395View attachment 170394
The matches were a home game on the 13th (away game to Kansas was the match before) followed by two away trips to Colorado and Seattle and Felipe, who has a Natural Fitness of 8 and the only player in the starting 11 to have one below 10, goes Match Fit to Tired. He's played fewer matches than Dax running almost 2km on average per 90min less than Dax at the time of the first screenshot. At the time of the second screenshot Dax ran an extra 23 km with 269 minutes played, while Felipe ran 15 km in 220 minutes. Dax' Jadedness is at 47 up from 39, while Felipe's is 89 up from 44. If it continues, Felipe won't be match fit for games soon. I don't know the cut offs for when you start seeing noticeable decreases in Condition recovery from Jadedness though.
I'd imagine that being the Dutch nat'l team manager and a club manager in Holland, you may end up showing favouritism to your club players thereby increasing their match load. Since you leave training to the assistant, you have to check his "Hardness of Training" attribute as well as the workloads given to each player.
A good way to have a good idea of Jadedness is managing in the USA for a bit. Those cross country away trips, wonky schedule, and continental games, and style of play are brutal on Condition. Especially if you're a Canadian MLS team.
Edit: Most of this comes from my observations and the Guide to FM site.
