They weren't winning either way, but not closing with Dieng and sitting Mills for a spell mid 4th robbed them of their best chance. Sure, a 13-14 minute and 18-19 minute stint for both would have inevitably left them out of gas for the finish, but they desperately needed all the shooting they could muster, plus Dieng at least provided some resistance for Valanciunas on the glass.
Poeltl's offensive limitations combined with his inability to adequately defend overpowering post up behemoths, makes re-signing Dieng a necessity.