Game time is the most important thing to getting improvement. With your Center, and only 2 years left, it's good that he has such a high work ethic. I run a M2M defense and often I will use my PF and C interchangeably for defensive matchups. With your 2, the PF being clearly the better defender, you could match him up against the better offensive big man for the other team. However, If you'd like to put them in there and keep them at one position, I'd keep the C at C and the PF at PF.