Hi @ Fabien le Mauff
If you want, check my guide for fast trainers (it's in my signature). It has many examples but consider that
fast trainers are expensive and if you 're playing with the sources of the game only, it's better to use other tactics than spending all your budget for only one player.
Also, fast trainers earn their money if you give a lot of training to them so they can become 6* or 7* players
- but this need extra green packs and red packs if he got injured.

So, if you have some extra, try them, it's worthy. If don't, try to get some veteran 5* players, over 27 y.o. who are cheaper and maybe some times they are better players.