Parsons -- whose father is a military vet -- was irked after finding out Whelan wasn't part of the deal to bring BG home ... saying, "Wait nah!! We left a marine?!! Hell nah."
The 2021 first-round pick clarified he was thrilled Griner was on her way back to U.S. soil ... but explained, "I just have family who have served and it’s crazy to me the President wouldn't bring him home too."
"Also if what I'm told about the attempts to bring Ret Marine Paul Whelan home are true then the best outcome was accomplished. I pray Mr. Whelan comes home but am extremely happy for Brittney and her family."
"I am not too prideful to admit when I've made a mistake."
The White House said the U.S. tried like hell to include Whelan in the Griner deal ... but ultimately, Russia wasn't going to budge.