The 49ers general manager confirmed the report.
"So, he is going to have surgery," Lynch said. "They got a couple of opinions on the groin. It's a core muscle injury that Dre's been dealing with here for the last couple of weeks, and he did a great job kind of battling through it. [He] had a big impact on the game against Detroit.
"It was actually on the pick-six [off quarterback Jared Goff] that he kinda re-tweaked it, and at some point, we scanned it, and this core muscle injury just continued to get worse. It got to the point where we felt the best interest for everyone was to go ahead and handle it now."
Barrows reported that Greenlaw is expected to miss four to six weeks. Later, NFL insider Ian Rapoport reported that the linebacker was visiting with Dr. William Meyers in Philadelphia and would miss six to eight weeks. Lynch explained why the recovery timeline can't be determined just yet.
"As for the timeline, I think that's best left until they get in there and see the extent," Lynch said. "But I think [the 4-to-6-week estimate is] accurate in terms of initially what we know and think. But I think it's always best to kind of wait on that. But we'll see where that goes."
Greenlaw is the third starter the team has lost as a result of Week 1's victory over the Detroit Lions. In addition, cornerback Jason Verrett suffered a torn ACL while running back Raheem Mostert will undergo a procedure on his knee. Both players will miss the remainder of the season.
"It's a loss, to say the least," Lynch said of the Greenlaw injury. "The good news is that all indications are we'll have him back later this season. We look forward to that. We'll have to hold the fort down until then."
You can listen to the entire conversation with Lynch below.