Why?

That’s the big question on the mind of Yankees fans.

Why would left-hander James Paxton wait until the week before spring training begins to have back surgery?

The Yankees announced Paxton went under the knife Wednesday and will be out 3-to-4 months, meaning his 2020 MLB debut might not come until June.

General manager Brian Cashman explained the decision-making behind Paxton’s surgery.

“He pitched extremely well, especially in Houston (in the American League Championship Series),” Yankees general manager Brian Cashman said, per ESPN. “But when he started ramping up his winter program about four weeks or so ago, he started feeling symptoms again.”

According to ESPN, Paxton tweaked his back in his final regular season start in 2019. The Athletic reports an MRI was performed and Rangers team doctor Keith Meister and Yankees team doctor Christopher "concurred that Paxton would be fine to receive an injection and proceed as normal into his first postseason appearance.”

But once (Paxton) began his offseason throwing program, he reported he felt pain in his lower back again, according to Cashman. At that point, Paxton visited Dr. Andrew Dossett in Dallas for what was now a third opinion, and was given a similar recommendation: Try a different version of an injection and manage it conservatively for now.

“It turned out clearly that that did not resolve the problem,” Cashman said, per ESPN, “so surgical intervention then was the last resort.”

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Paxton leaves a huge hole to fill in the starting rotation. He is arguably the Yankees’ second-best starting pitcher, behind the newly signed Gerrit Cole and either neck-and-neck or just ahead of Luis Severino.

In his first season in the Bronx, Paxton went 15-6 with a 3.82 ERA in 29 starts. But he looked like ace material over his final 11 starts, winning 10 of them with a 2.51 ERA while striking out 69 hitters in 61 innings. He had a rough time in his first two postseason starts, but stepped up big-time in Game 5 of the ALCS, holding the Astros to one run over six innings.

(NJ Advance Media’s Brendan Kuty contributed to this report.)

Mike Rosenstein may be reached at mrosenstein@njadvancemedia.com. Follow him on Twitter @rosenstein73. Find NJ.com on Facebook.