Where will Celtics turn after Al Horford leaves for Sixers?

Despite the imminent signing of Kemba Walker by the Celtics, it was still a tough start to free agency for the green on Sunday. Hours after Kevin Durant agreed to join Kyrie Irving in Brooklyn, Al Horford came to terms with the 76ers on a four-year deal worth $97 million, with another $12 million in possible championship incentives, according to a league source. Adrian Wojnarowski of ESPN.com was first to report the news.

The 76ers will be revamping their starting five for next season after also agreeing to a sign-and-trade of Jimmy Butler to the Heat for Josh Richardson. With Tobias Harris also re-signing for five years, Philadelphia now has the biggest starting five in the league with Ben Simmons, Josh Richardson, Tobias Harris, Al Horford and Joel Embiid.

The Celtics, according to sources, were in the mix for Horford today but did not come close to matching the guaranteed money that the Sixers offered. The Celtics could have maintained Bird Rights on Horford if they managed to pull off a three-team sign-and-trade with the Nets and Hornets as broken down here. Boston would have needed to include at least one first round pick to get the deal done (to compensate Brooklyn) and possibly more. From that perspective, keeping Horford would have cost at least $24 million per year for Boston as well as some draft compensation (for Brooklyn in the sign-and-trade).

It appears that was too rich of a price to pay for Danny Ainge and company. Horford lands in a perfect situation for him, allowing him to return to his natural position at power forward next to Joel Embiid. He gets paid and will have the ability to contend in the East for the foreseeable future, with (arguably) more talent than every team in the East beyond the Bucks (depending on Kawhi’s decision). The Sixers are also a huge team now and without a true point guard. Losing JJ Redick will hurt their offense a lot so it’s not clear whether all of these pieces will fit well. Brett Brown has his work cut out for him but Horford is a nice insurance policy for Joel Embiid that should limit some wear and tear on the big man.

The question now for the Celtics is where they go from here. A sign-and-trade involving Irving or Horford could still be negotiated for Walker which would allow the C’s to stay above the cap and have access to the full mid-level exception and keep Bird Rights on Marcus Morris. The problem for Ainge is that the C’s will only have two division rivals to negotiate with on that front. Will it be worth giving the Sixers or Nets a first round pick to gain access to the full mid-level exception? Or will the difference in talent level not be worth the cost? That’s a decision the front office will have to make in the coming days as their options crystalize more in the middle.

The problem for the Celtics is they didn’t just lose their best players, they lost them to two rivals that will have no interest in helping them with a sign-and-trade. Both teams have the cap room to sign Irving and Horford outright, which puts Boston in a tough spot. The Sixers were able to leverage an asset out of Miami (Richardson) since he wanted to go to a team without cap room. The Celtics would have loved it if Horford picked a team like that (so they could get something out of it). They were unlucky on that front.

The road back to contending just got a little bit tougher now in the East for the foreseeable future. The Celtics still have some solid options at center available as of Sunday night (Enes Kanter, Robin Lopez (signed with Bucks), Kevon Looney, DeMarcus Cousins) but they have a huge hole to fill in the middle to make up for the loss of Horford and Aron Baynes. Whether or not they have $4.7 million or $9 million to throw at those names will become more clear in the coming days when sign-and-trade scenarios pan out or not.

For now, a window of opportunity closes to some degree for Boston and a bet on the team’s young talent only grows. If Leonard departed to LA, the case could be made for the Celtics being a top contender in the East next to the Bucks if Horford returned. Boston wasn’t willing to pay full sticker price for the 33-year-old in what may end up being a wise move in the long run.

For now? The Celtics fall back further into the pack in the East, at least on paper. The front office maintained their assets and long-term cap flexibility for now by passing on Horford, but they also passed on their best chance of being a contender next season. With Kevin Durant and Klay Thompson sidelined for most of next season, the door is open for more teams than ever to make a run to the NBA Finals. The path for Boston to get back to that level is going to be a tough road to navigate with the talent level in the East increasing by the day.