Green was placed on long-term injured reserve by the Capitals last week.
"It's basically a sports hernia, but there are a couple parts to it," McPhee said during the Capitals' morning skate ahead of Tuesday night's game against the Islanders. "One was a tendon issue, one was a little tear in the stomach (lining). The tendon healed just fine, but the hole didn't close up. We just need to close that thing up. It just never really healed to the point he can play with it effectively."
Green initially suffered what was considered a groin injury Nov. 11 at New Jersey. It was his first game back after missing six straight games with an ankle injury.
SOG: 20 | +/-: 1
McPhee said surgery was not considered necessary after Green suffered his initial injury in New Jersey more than two months ago.
"You always want to take the conservative approach," McPhee said. "We did, and parts of it went real well and one part didn't go so well, so we need to repair the one area. It will now eliminate the unknown. Sometimes when you take the conservative approach, it takes a while and you don't really know when the player is going to be ready. With this procedure, we know when the player should be ready."
McPhee said Dr. Blunt has performed this type of surgery on former Capitals players Jamie Heward and Chris Clark.
"We think he's the best at this particular surgery," McPhee said. "Our medical staff obviously consulted with him early on with Mike. The consensus of the group and the doctor was let's go conservative, initially. And that didn't work out, so now you get a more aggressive approach. Hopefully there's some finality to it and he's ready to go in four to six (weeks)."
Green has played in only 10 games this season and the Capitals have won nine of them. Going back to last season, Green has appeared in only 12 of Washington's last 72 games due to a variety of injuries.
Green is a restricted free agent following the season. He is in the final year of a four-year contract that pays him $5.25 million annually.
McPhee also said center Nicklas Backstrom is progressing in his recovery from a concussion and could return before the All-Star break. Backstrom will miss his sixth straight game Tuesday since being flagrantly elbowed in the head by ex-Flames forward Rene Bourque on Jan. 3. Bourque, who is now with the Canadiens, received a five-game suspension.
The Capitals will face Bourque and the Canadiens on Wednesday in Montreal.
"Whenever he's ready to get going we'll get him going," McPhee said of Backstrom. "But obviously we're going to be real careful because we want this to be a one-time thing and something we never have to worry about again."
Follow Dan Rosen on Twitter at: @drosennhl