BOSTON (CBS) — Thursday is the most important day in Boston Celtics history.

Yes, that is a lofty overstatement. But the balance of the Celtics’ season hangs in a medical office somewhere as Celtics guard Kyrie Irving is heading to get a second opinion on his sore left knee, according to the Boston Herald’s Steve Bulpett.

Irving has missed the last four games as he deals with lingering soreness in his knee, and Thursday’s exam will determine if he can return after some more rest or if he’ll have to undergo a procedure to remove screws in the knee. That procedure would force him to miss the rest of the regular season and the playoffs, and pretty much end any chance at a deep postseason run by Boston.

The soreness comes from the fractured kneecap Irving suffered during the 2015 NBA Finals. He was expected to require a procedure on the knee after the season, but the soreness may bump up that timetable.

“He’s out because of knee soreness, not because we’re choosing to rest him,” Celtics head coach Brad Stevens told reporters ahead of Tuesday’s win over the Oklahoma City Thunder. “That’s the bottom line. Again, we want him to feel 110 percent. He wants to feel 110 percent. Obviously, we’re fortunate we created a cushion early on in the year with playoffs and everything else. This is not one of those situations where we’re choosing to rest someone; it’s because he has a sore knee.”

Stevens added that Irving has had some good days recently, and they are confident this won’t linger into next season. But the rest of the 2017-18 season ultimately comes down to Thursday’s second opinion.

Hold on tight, Celtics fans.