Firstly any player you get from a pack is automatically loyal so for this achievement you will need to buy a player from the transfer market.
The fastest way to do this would be to start a new season and forfeit the first 9 matches. The achievement will pop when you start the 10th match.
- you can tie this to the win a FUT title achievement by forfeiting the first 6 matches and winning the final 4.
- make sure the player has enough games on his contract.
- if you don't want to waste players contracts use players you don't need for the rest of the team.
- you can forfeit as soon as the match starts.