When Gordon Hayward went down with a visibly devastating injury in the first game of the regular season, Boston Celtics fans knew this wasn't going to be their year. But then a funny thing happened: They started winning ... and kept winning ... and kept winning.

Suddenly, in a down year for the Cavs, Boston looked like a possible favorite to come out of the East, even without Hayward. Then Kyrie Irving went down -- surgery to clean up a previous surgery. Now, for sure, the Celtics would be done, right? Nope, not this time either. They've gone 7-4 in Kyrie's absence, including wins over tough Toronto and Utah teams, and Boston fans were once again thinking that they could do some damage in the playoffs once Irving made his triumphant return.

On Thursday, however, we found out there will be no triumphant return. Irving is done for the regular season and the playoffs due to another knee surgery, leaving the Celtics to see how far this ragtag group can make it in the postseason. 

Here are five things to know about Irving's season-ending injury.

So, uh, who's going to score?

Irving's teammates have given a valiant effort in his absence, but the numbers support the eye test: The Celtics have struggled to score with Kyrie out of the lineup. Since Irving went down in mid-March, the Celtics' offensive rating has dropped from 105.7 to 101.9. Their defensive rating has stayed basically the same, which has allowed them to remain competitive, but the playoffs are a different animal. When opposing defenses ramp up and are game-planning for players like Jayson Tatum, Marcus Morris and Terry Rozier in a seven-game series, who's going to be able to put the ball in the basket? Al Horford prefers his role as a facilitator, but at times he's going to have to take over as a scorer if the team's going to win playoff games.

Boston's focus officially shifts to next season

The Celtics have had every excuse to chalk 2017-18 up as a lost season, but they've continually defied the odds, which will likely earn Brad Stevens his first NBA Coach of the Year award. Fans, and even the team, could convince themselves that they had a real shot at making it to the NBA Finals this season as long as Irving came back healthy for at least the second round. With that out the window, it's officially time to look toward next year. With a (hopefully) healthy Irving and Hayward back, to go along with a young core that will gain valuable playoff experience doing things on their own over the next few weeks, Boston will almost undoubtedly start next season as the favorite to come out of the East.

Tatum can become a star

Thanks to Hayward's injury, Jayson Tatum was thrust into a more prominent role in his rookie season, and he's performed brilliantly. Now, with Irving out, he's going to be relied upon as a go-to playoff scorer -- something the 20-year-old is more than capable of doing eventually, but can he do it this soon? He'll probably have his ups and downs with the other team's best perimeter defender on him on most nights, but if he can carry the Celtics to a few wins he might just establish himself as an NBA star well ahead of schedule.

Legend of Stevens can go nuclear

Stevens is already the darling of the NBA coaching fraternity and a whiteboard wunderkind the likes of which we haven't seen in a long time, but if he can lead this team -- not only without Hayward or Irving, but also without key role players like Marcus Smart and Daniel Theis -- even to the conference finals, they might just enshrine him in the Hall of Fame this fall. Whether it's running Tatum at point guard or implementing a 2-3 zone defense, Stevens has had to push a lot of buttons to get the Celtics to the No. 2 seed this season, and he's going to need some more tricks if they're going to stay afloat during the playoffs. Which brings us to our next point ...

Don't count the Celtics out

Say what you will about the lack of firepower and inexperience on the roster, but the Celtics are almost impossible to blow out because of how well they defend. That's not going to stop in the postseason, which likely means Boston will be close at the end of most, if not every game of their playoff run. When a team can keep things close and executes the way the Celtics do, they always have a puncher's chance, even against the best teams in the league. The Raptors are struggling heading into the postseason and nobody quite believes their transformation, we don't know how this new Cavs construction will perform in the playoffs, and the rest of the East is plagued by either injuries or playoff inexperience. Things are as open as ever and it would be foolish to count Boston out, even without their two best players.