Two weeks ago, after Thunder guard Andre Roberson sustained a season-ending ruptured patellar tendon in a win over Detroit, Warriors forward Kevin Durant shot him an encouraging text.

“I let him know I’m here if he ever needs to talk or anything because I’ve been through it, you know, injuries and how you feel when you’re going through them,”said Durant, who played with Roberson in Oklahoma City for three seasons (2013-16). “Just try to give him somebody else to lean on if he needs some advice or just some encouragement. I’ll always be here for him.”

It should come as little surprise that Oklahoma City has struggled mightily on defense without Roberson. In five games without Roberson, the Thunder have gone 1-4, allowing an average of 112.6 points.

Though hardly an offensive dynamo, Roberson is Oklahoma City’s best all-around defender. The reigning All-Defensive Second Team selection often guards the opponent’s top scorer.

Per ESPN Stats & Info, in the 539 minutes Roberson played alongside Russell Westbrook, Paul George, Carmelo Anthony and Steven Adams, the Thunder allowed 95.9 points per 100 possessions. Westbrook, George, Anthony and Adams allowed 114.5 points per 100 in 402 minutes without Roberson before Roberson got injured. Roberson’s defensive real plus-minus of 4.34 ranks second in the league, behind only Miami’s Hassan Whiteside.

He’s “probably the best” defender in the league, said Durant, who is an early candidate for Defensive Player of the Year. “His lateral release, his quickness. He’s got long arms, and he’s strong. He’s just smart. He’s just an athlete, one of those athletes who enjoys trying to keep up with the fastest and best players. He takes that challenge on. They’re definitely going to miss him on the defensive side of the ball.”

Roberson’s absence could be particularly glaring Tuesday. A master at switching off screens, he has often shouldered a heavy defensive workload against Golden State, matching up with everyone from Durant to Stephen Curry to Klay Thompson.

George, who leads the NBA with 2.16 steals per game, will be asked to handle much of that responsibility.

“You know, (George) is a great wing defender as well,” said Warriors forward Draymond Green, the reigning Defensive Player of the Year. “I think without having (Roberson), them still having PG helps a lot.”

Connor Letourneau is a San Francisco Chronicle staff writer. Email: cletourneau@sfchronicle.com. Twitter: @Con_Chron