To sell or not to sell?

[Anquilis]

90% is not that much, not at all. It is possible to train the team up to 130%+ without paying real money (you need to be in Gold1 or better in Platinum though)
21 years old is almost an old man. Not yet, but close.
The offer is very good. I'd sell and buy 18 y/o FT instead.

Thanks man, but what do you mean by FT?

[Toxcatl]

Thanks man, but what do you mean by FT?

Fast trainer. A player who progress faster than others. Have you noticed that two players of the same quality may have different Value? That's because there is a hidden parameter that determines how fast the player gains skills.
Check this table. How to use it. For example, you've found a player q55. In the table there is a row with q54, max Value is 17-19m. So your player q55 to be a good FT must have Value 18-20m, approximately. If he is significantly cheaper, he is not a FT.
The table is for 18 y/o players only!

The general strategy is simple. You have to squeeze green packs from everywhere. Watch ads, make subs, don't use 2x bonus training. In the beginning of every season buy up to three 18 y/o FTs to replace your oldest and weakest players, train them until 7*, and only after that train others or save packs. Repeat every season, and after four seasons you will have a team of 18-21 y/o high-quality FTs. There are some details and exceptions, but I guess you got the main idea.