Klay was joining the guys on TNT's "Inside the NBA" during halftime of the Warriors vs. Clippers game ... where Golden State was getting smacked around by L.A.

Of course, Klay is still out recovering from the torn ACL he suffered during the NBA Finals ... but he told the guys, he's still optimistic the Warriors can get back to dominating the league.

The issue ...  without Klay and Kevin Durant (who left for Brooklyn), Barkley thinks the team is in big trouble.

"Our window to win another championship I think is open for the next 5, 6 years," Klay said.

"Number 1, I hope you get back soon but you're just wrong," Barkley shot back.

"I think their window's shut. I don't think they can score enough."

Barkley also amended a prediction he made before the game -- "I was wrong before the game. I said [the Warriors] were gonna be the 7, 8 seed. They're not gonna make the playoffs."

Kenny Smith asked, "You know after one half of basketball?!"

Barkley responded -- "Yes."

Klay tried to defend his squad and said he has faith in the young player on the Warriors, but it was clear Barkley wasn't convinced.

By the way, the Warriors lost 141 to 122 in their first home game in their new arena.