Special trainer is the worst deal of T11 as Gert saying. The only case for using him is if you have an old player (28-30 y.o.) were power-training has little effect to him and of course a LOT of token.
The best options for you are :
- A young fast trainer the first day of the season, after the daily maintenance with position ST or AM.
You 'll buy him, test him in old version training and if he gets over +100 invest to him.
Use him in F-K in a couple of friendlies or in the Cup game.
If he scores, give him F-K specialist and set him kicking all the F-K (and penalties too)
The talent at F-K is the key to be a star.
Make him ST- AM_
- An academy player, the expensive one. Also with the same procedure above
- A recommended player ST-AM_ , 19 y.o.

The rest depending from how many greens you have. I can farm daily 26-30 greens (- 2 for using the X2 training bonus), so with this amount I can keep a 6* team, early 7* at the end of the season.
With spare greens, you can do power training to your Star, so to have him 1* more 7*, early 8* at the end of the season.
An 8*-9* player among a 5* team can't help much.
The team could not create many chances for scoring, keep oppo away, blah blah
In a 6* team, I believe he can shine.
* It's not necessary to have a full team of 18-20 players at 6*, only 10-11 the basic.