Now that we’ve had some time to digest what happened to the Boston Celtics during the regular season, it’s time to look forward to see what might happen to them in the future. That begins with a look at their salary cap situation.

So here’s a look at where the Celtics stand with their salary cap, and what they might have available if Kyrie Irving decides to leave.

The basics

The NBA salary cap will be approximately $109 million next season. The NBA has a “soft cap,” which means they can go over the cap in certain instances to sign players.

The luxury tax threshold will be approximately $132 million. Any money a team spends above that will be taxed. Taxes start at $1.50 per dollar the team is over the tax threshold and increases 25 cents every $5 million of additional salary. A repeat offender (any team that has paid the tax in three of the previous four seasons) starts at a tax rate of $2.50 with the same increases.

The Celtics are currently guaranteed to pay to about $121.5 million in salary, but that’s nowhere near the final number.

Some terminology

Here are some terms you’ll hear from time to time. They’re important to understanding why certain things can and can’t happen. You can skim by this if you want and refer back to it if something comes up later that you don’t understand.

Player option: This is a salary the player can either take or turn down. The team has no say in this. Kyrie Irving ($21.3 million), Al Horford ($30.1 million), and Aron Bayes ($5.4 million) all have player options.

Non-guaranteed contract: A contract that can be voided by the team by a certain date. Semi Ojeyele has a $1.6 million non-guaranteed contract. The Celtics have until July 1 to decide if they’ll guarantee it, which would mean he gets paid the whole amount the upcoming season. If they don’t guarantee it, he becomes an unrestricted free agent.

Unrestricted free agent: A player without a team. He can sign anywhere for however much. The former team has no say in the player’s decision.

Restricted free agent: A player who is free to sign an offer sheet with any other team, but his current team has the right to match the contract and retain his services. This is almost always a player still on a rookie deal, like Terry Rozier. So these players can sign with another team, but their current team can say “we’ll match that contract” (which they have to do, word for word) and keep the player.

Qualifying Offer: A team has to make the “qualifying offer” to a restricted free agent to kick in their right of first refusal. Once that offer is made, the player is a restricted free agent.

Bird Rights: This is a mechanism that allows a team to go above the cap to sign its own free agents. It can get complicated, but in a nutshell, if a player has been with a team for three years or more, the team has his Bird Rights. They are transferable in a trade, so Boston has Kyrie Irving’s Bird Rights.

Cap Hold: This is a fake number that prevents a team from getting tricky with Bird Rights. It serves as a placeholder for what a team might pay a player to prevent teams from using their cap space to sign a bunch of free agents and then signing their own guy. For example, the Boston Celtics have a $10 million cap hold for Marcus Morris. It exists on their books until he signs with another team, they sign him (and it’s replaced by the actual contract number), or if the Celtics renounce their rights to him. The Celtics could still sign him after doing that, but he’d be like any other free agent and Boston would have to use cap space or an exception.

SIDE NOTE: These apply to draft picks as well. Once it’s determined which picks Boston will get, holds for the pre-determined salary slots will be placed on Boston’s cap sheet.

Now to the nitty gritty: What if Kyrie Irving leaves?

There are a lot of moving parts to this team, and there’s no simple answer to what happens when one player leaves. Kyrie Irving’s departure wouldn’t just open up a $21 million slot for Boston.

One assumption I’ll make here is that Boston isn’t making a trade in this scenario. The whole purpose of this particular piece is to figure out how much money Boston might be able to spend on a free agent if Irving leaves. I’m going to pretend, for the purposes of this particular piece, that Danny Ainge is going to roll with the guys he has if Irving leaves.

Another assumption that I’m going to work with here is that Al Horford is either going to leave outright or opt out and sign long term. He can get a five-year deal that exceeds any kind of money he’d get in free agency, so if he stays, he’ll stay for good.

This requires some guessing on my part, so I’m going to guess a five-year contract worth $90-$95 million works for both sides. Yes, he’d give up $30 million next season, but he probably won’t make $60 million over the final four years of his career, so this is a long-term play for him. His first-year salary in this scenario would be $17 million or so.

If Irving and Horford leave, then Boston’s guaranteed salaries for the seven remaining players would be $70 million.

Of course, he might opt-in, so if Irving leaves and Horford stays for $30 million, Boston’s guaranteed salaries would equal about $100 million for eight players.

If Irving leaves and Horford takes my long-term scenario, the guaranteed salaries would equal about $87 million. At this point, the roster would be:

Gordon Hayward

Al Horford

Marcus Smart

Jayson Tatum

Jaylen Brown

Aron Baynes (opting in to his $5.4 million)

Guerschon Yabusele

Robert Williams

That’s eight players. Boston would need to fill its roster out with six more players.

Boston will get at least three first round picks this year, maybe four. We know of three for sure so we’ll roll with that number. Those picks, assuming they are at their projected slots of 14, 20, and 22, are guaranteed to make $7 million. Boston always gives picks the extra 15 percent they’re allowed, so adding three draft picks to the mix (assuming they are all on next year’s roster) equals $8.4 million in guaranteed salary.

That brings Boston’s roster up to 11 in the “Horford stays long term” scenario, giving Boston four more open spots if they go to the 15 player max. The guaranteed total becomes $95.4 million.

That doesn’t mean Boston has $14 million in cap space. There is currently a $10 million cap hold for Marcus Morris and a $9.1 million cap hold for Terry Rozier. There are other smaller ones, but these holds keep Boston over the cap unless they choose to renounce these players.

Here’s where the decisions get tough.

Boston can go ahead and renounce their remaining free agents and not guarantee Ojeleye’s deal and have $14 million to spend on free agents, plus a “room mid-level exception” worth somewhere around $4.5-$5 million.

It might be better, though, to re-sign Morris and see if he takes three-years, $30 million or so to come back. They can guarantee Ojeleye’s deal to fill out the bench. They might even decide to bring Rozier back on a more team-friendly deal and roll with a starting lineup of Rozier, Hayward, Tatum, Brown, and Horford with Smart, Morris, and Baynes coming off the bench.

This scenario would also give them a taxpayer mid-level to use, probably somewhere close to $6 million, to use on a solid bench player to fill out the roster.

You might not like that scenario, which is fine. It’s just a theoretical possibility. The lesson, though, is that Kyrie Irving leaving doesn’t mean Boston gets to sign a big free agent. It doesn’t even mean they get to sign a great free agent.

Even if Irving leaves and Horford helps out with a deal that cuts his salary in almost half next season, the Celtics won’t have much money to throw around. Unless they make a trade to re-shape their roster, they’ll have minimal money to do much of anything.

This is really why it’s in Boston’s best interests to have Irving back. There’s no way to fill his shoes if he leaves, unless you believe in addition by subtraction.

The best-case scenario is that Irving re-signs and spends a summer stewing over the hard lessons he learned and coming back a stronger leader. That gives Boston the most talent and best chance to win.