Congratulations on the league title Swamp things and best of luck in D.3
As for Donny, your stud draft pick, I think it depends on what you want your team to do over the next few seasons. Since my kitties just came down from D.3 last season, I would say that your team is currently a bubble team for staying up in D.3. Think you need some upgrades to better compete in D.3. It also depends on if you have another few seasons in which you can train your current bigs.
If you 1) want to stay up; 2) have some cash in the bank; and 3) can continue to train your bigs for a couple more seasons, I would sell Donny for some serious cash and use that cash, plus what is in the bank, to upgrade a guard, preferably both guards.
If you 1) don't mind a short stay in D.3; 2) don't have a lot of cash on hand, and 3) your bigs are nearing the end of their training cycle, I would train Donny up to be a star, but know that it will be a long D.3 season for you and you will end up back in a D.4 league. If going this route, I would sell some other players and get another stud guard trainee to train up along side Donny, to form a killer back court down the road.
As your promotion to D.3 was hard earned, I think option #1 is the better one, but your call. Good luck!