Kevin Durant thinks Perkins was a trash player who constantly underperformed.
... This was just the beginning of an epic Twitter war between the former NBA teammates on Thursday -- and it's clear, the beef is REAL.
It all started with Perkins saying Russell Westbrook is the best OKC Thunder player ever -- even calling Russ, "Mr. Thunder!"
When NBA analyst Marc D'Amico challenged Perk, saying KD has the better resume, Perk took another swipe at Durant.
"KD lost in the second round without Russ when I was there. So what that mean?"
That's when Durant got involved -- clapping back HARD at Perk.
"Yeah and our starting center [Kendrick Perkins] averaged a whopping 2 and 3 during that series. U played hard tho champ lol."
Perk didn't back down and responded again ... burning Durant for leaving OKC to go to the Warriors.
"Boy stop you did the weakest move in NBA History!!! Up on a team 3-1 in the western conference finals and then go join them the following season?! Heart of Champion right there."
Again, KD clapped back ...
"Weak is starting at center, playing real minutes with no production. Should've worked on your skills as much as I did."
"That's fine!!! You worked that hard and still had to go join a 73-9 team. Truth be told, you don't even feel like a real champ, you have hard time sleeping at night huh knowing that you took the coward way out!!!"
That's where the tweets stopped -- but we're guessing this ain't the last we're gonna hear from either side about this.