TAMPA — The lingering issue Steven Stamkos has been dealing with isn’t going away soon. He will have surgery for a core muscle injury and is expected to miss six to eight weeks, the Lightning said Saturday.That time frame will keep the forward, second on the team in goals and points, out into the postseason. Six weeks could be around Game 3 of the first round, and eight could be in the second round.The surgery was scheduled to be performed Monday by Michael Brunt at Washington University in St. Louis. Brunt specializes in minimally invasive abdominal surgery, the school’s website says.“Any time you lose as big a part of your team as your captain, it’s a loss,” coach Jon Cooper said. “You can’t replace him. He’s one of the best in the world at what he does.”The Lightning came up with a plan for Stamkos after consulting with multiple physicians and a “very thorough” evaluation,” Cooper said. Nobody wants to shut a player down with 17 games remaining in the regular season, but a team has to do what’s best for the player.This is the fourth time in his 12-season career that Stamkos, 30, will miss substantial time because of health problems. One was at the end of the 2015-16 season, when he had surgery for a blood clot near his right collarbone, and the Lightning made it to the Eastern Conference final without him.No one announced the surgery to the team. Players found out organically before the news went out to the public. Most players reached out to Stamkos when they heard.Stamkos was in Amalie Arena on Saturday for the game against the Flames, but he and general manager Julien BriseBois were unavailable to the media. Stamkos did not present any obvious signs of mobility issues. Stamkos left Tuesday’s loss to the Maple Leafs after the second period. At the time, Cooper said a lingering issue he’d been able to play through had cropped back up. The Lightning wasn’t sure exactly what that meant for Stamkos until he was evaluated Friday.Stamkos missed three games earlier in February. The surgery is not for that injury, but it’s unclear if that issue and the core muscle injury are related or if looking into the first injury revealed the second.The situation could be worse for the Lightning. At least there’s an expectation that he can return in the postseason. But this is still a major hit.Stamkos, with 29 goals and 66 points, was flirting with another 40-goal, 90-point season. Even while dealing with the issue, Stamkos had at least a point in each of his past 15 games played for a total of 12 goals and 22 points.His on-ice impact goes beyond goals, however. Stamkos is the kind of player teams specifically plan around. When their schemes key in on him, it creates openings for others.“It’s huge,” forward Yanni Gourde said of the loss. “Steven has been such a huge player for us throughout the season. The way he plays, his leadership, we’re going to miss him, for sure.”The power play has been struggling the past two months, going 8 for 70 since Jan. 1. Now it just lost its biggest goal-scorer. Stamkos has 10 of the Lightning’s 46 power-play goals.Stamkos’ captaincy is more than a symbol. He is the true leader of this team, and that voice will be missed as much as his goals. Gourde said players will have to each step up, that it’s unlikely any one player will fill that void.This is where the Lightning’s depth is crucial, especially down the middle. They have so many centers that Stamkos has played wing much of the year, though he’s still the team’s best player in the faceoff circle.Brayden Point has consistently been the top center and Anthony Cirelli the second this season. Tyler Johnson and Barclay Goodrow can share the role on the third line. Cedric Paquette and Mitchell Stephens figure in on the fourth.Stamkos’ injury solves any question of where Blake Coleman plays, though not the manner in which anyone wanted to get an answer. With Ondrej Palat on the top line with Point and Nikita Kucherov, Coleman slots in on the second with Cirelli and Alex Killorn. Contact Diana C. Nearhos at firstname.lastname@example.org. Follow @dianacnearhos .