Lars Sullivan reportedly underwent successful surgery earlier this morning, according to Wrestling Observer Live.
As noted earlier, Lars could be out of action for the rest of the year after suffering the knee injury. It was reported that Lars could be out for 6-9 months, which could mean he will be back in December or around WrestleMania 36 season, and that was still the word coming out of surgery, according to Bryan Alvarez on WOL.
There's still no word yet on when Lars suffered the injury but it was noted on Wrestling Observer Live that the injury was a "fluke deal" and apparently occurred when The Freak was just doing something routine. The injury was discovered to be much worse than originally believed last week.
Sullivan last wrestled The Lucha House Party on last week's RAW following his win over them at WWE Super ShowDown in Saudi Arabia the Friday before. Lars was set to face Matt Hardy on last week's SmackDown, but the match was nixed due to the injury.
On a related note, Lars recently re-activated his Twitter account. It was de-activated in late May. Lars has not made any tweets since early May.