On standard player growth every player has set development curve so some players will naturally decrease at a certain age, if someone sits on the bench this can also effect growth but in drogba's case its probably just his natural curve.
You could start a new master league and try dynamic player growth which is more focused on performances rather than age and a set development growth.
For eg if you used dynamic player growth and drogba was in top form his stats would increase, if playing badly his stats may decrease.