I recommended going for other achievements while trying to get this one. To unlock a pro player you must buy card packs and get every card of the certain team that player is on. For example, to unlock the drew brees pro player card you need every regular card of the Saints team. So to do this you must buy card packs with your Blitz Bucks. What I recommend is playing Blitz Battles to work towards getting the head to head and Blitz Team battle boards achievements and using the Blitz Bucks you earn from there to go towards card packs. I got my Elite League team to a record of 36-5 before I realized I was wasting time with those games. GOOD LUCK
EDIT: Once you complete a team of cards do NOT trade it in right away. If you do the game will start giving you the cards for that team again since you technically don't have them anymore, making it take longer to get what you actually need.