As noted, Lars Sullivan underwent successful knee surgery on Thursday morning and is expected to be out of action for 6-9 months.
Lars apparently had been working injured as he was dealing with a minor injury going into the recent RAW match against The Lucha House Party, which was the rematch from WWE Super ShowDown. It was noted on Wrestling Observer Radio that Sullivan already had a minor injury, but it was worsened two weeks ago when he did the Handicap rematch on RAW.
The RAW match against Lucha House Party was the last time we saw Lars on WWE TV. He was scheduled to face Matt Hardy the next night on SmackDown, but the match was nixed due to his injury.
Lars could be out of action until this December or some time around WrestleMania 36 season, which was the word coming out of Thursday's surgery. The injury to Sullivan has been described as a "fluke" or a "freak accident."