Here’s a little thought exercise.

Think back, less than three years ago, when the Celtics were opening the 2014-15 season. The starting lineup was Rajon Rondo, Avery Bradley, Jeff Green, Jared Sullinger and Kelly Olynyk. Marcus Smart, Evan Turner, Brandon Bass and Marcus Thornton were the first four guys off the bench.

By the time the 2017-18 season starts, it’s possible none of those guys are wearing green.

The Celtics have three cornerstones now: Isaiah Thomas, Al Horford and Gordon Hayward, who agreed to a four-year, $128 million contract on Tuesday. But they also have three key components — guards Marcus Smart and Avery Bradley and forward Jae Crowder — who have proven to be more than serviceable NBA players and helped lift Boston to the Eastern Conference Finals.

There’s one problem. The Celtics need to get rid of one of them.

In order to create room for Hayward’s max deal, the Celtics have already renounced Kelly Olynyk, who reportedly signed with the Miami Heat for four years and $50 million on Thursday.

Free agent Kelly Olynyk has agreed to a four-year, $50M-plus deal with the Miami Heat, agent Greg Lawrence tells ESPN.

— Adrian Wojnarowski (@wojespn) July 6, 2017

Beyond that, they will renounce the rights to Jonas Jerebko, James Young and Gerald Green, then waive Jordan Mickey, leave Guerschon Yabusele in Europe for another year and cut Demetrius Jackson whose deal is only partially guaranteed.

But you’re still a couple of million dollars short of being able to pay Hayward his $30 million. Even trading Terry Rozier leaves the Celtics $300,000 short.

That means one of those three is headed out the door to make room for Hayward. Here’s a look at the pros and cons of dealing each of them.

Marcus Smart

Pros: As of right now, he’s the worst basketball player of the three. While a defensive dynamo who can guard bigger players, his offensive game hasn’t come around after three years in the league. His career true shooting percentage of .480 is one of the worst in NBA history and he’s never even shot 37 percent from the field or 35 percent from deep.

Cons: He’s the youngest of the three at just 23 years old and the guy who could improve the most from here on out. He’s also the cheapest next season at just $4.5 million, after which he’ll be a restricted free agent and the Celtics could match any offer sheets.

A proposed deal: There may already be one in the works. ESPN’s Ian Begley reported the Celtics contacted the Knicks about Smart and may be asking for center Willy Hernangomez, a 6-foot-11 first-team All-NBA pick last year who costs just $4.7 million total over the next three years. Later, the New York Post’s Marc Berman confirmed the Knicks are interested in Smart. This deal should be a no-brainer if the Knicks are actually willing. Trading Smart before he gets expensive for a cheap center who averaged 16 points and nearly 14 rebounds per 36 minutes last year? And it even makes some sense for the Knicks, who will struggle to play Hernangomez next to Kristaps Porzingas in this smallball era, and already have Joakim Noah and Kyle O’Quinn at center.

Jae Crowder

Pros: He’s the guy whose position Hayward is taking. And there’s also back-to-back No. 3 picks Jaylen Brown and Jayson Tatum waiting in the wings. The Celtics even selected a guy who could be the next Crowder in SMU’s Semi Ojeleye in the second round of last month’s draft. He’s also never been one who could easily create his own shot or put the ball on the floor effectively.

Cons: He’s a very good NBA player, on a cheap contract, which is about as rare as Haley’s Comet in the league right now. Crowder is due just about $22 million over the next three seasons, or nearly $30 million less than Evan Turner will earn with Portland. Crowder shot a career-best 40 percent on 3s last year, finished with a true shooting percentage north of .600 and can guard three positions.

Potential deal: ESPN’s Adrian Wojnarowski and Ramona Shelbourne reported the Celtics and Jazz have discussed a sign-and-trade that would send Crowder back to Utah as a prize for facilitating the deal.

Avery Bradley

Pros: He’s just a year away from being an unrestricted free agent — he’ll earn just under $9 million in the final year of a four-year, $32 million contract that’s among Danny Ainge’s best deals — and commanding a large salary. The 6-foot-8 Hayward can easily slide over to shooting guard in larger lineups to fill his hole at shooting guard. Like Crowder, he’s never been a guy who could create his own offense.

Cons: He’s the best all-around player of the three. He can shoot and defend, hitting 39 percent from deep last year while averaging career-best 16 points and six boards a game. He’s more adept with the ball in his hand than Crowder and came up big in several postseason games.

Potential deal: It’s tough to find a team that’s under the salary cap, needs a shooting guard and would be willing to take on a potential one-year rental. With JJ Redick having signed with the 76ers, and with Chris Paul and James Harden in Houston’s backcourt, that takes away a couple of options. The Bulls might be interested in a replacement for Dwyane Wade, or the Nets might be willing to him in a salary dump.

Follow Brendan Kurie on Twitter @BrendanKurieSCT