Horford's hasty departure to the Philadelphia 76ers apparently irked the Celtics, who believed they would receive an earnest opportunity to convince the 33-year-old to re-sign.
The New York Times' Marc Stein reported on June 21 - nine days before the start of free agency - that multiple teams believed Horford had a four-year deal waiting for him once he was officially allowed to have contact with teams other than the Celtics.
Just 15 minutes after free agency began at 6 p.m. ET on June 30, news broke that Horford had agreed to a massive contract with the 76ers for a figure later reported to be $109 million ($97 million guaranteed) over four years.
Horford spent three seasons with the Celtics, earning his fifth All-Star nod and an All-Defensive second-team selection for his efforts during the 2017-18 campaign.
With over $3 billion in new contracts reportedly being committed to by midnight on June 30, several teams could have made inappropriate contract with other clubs' free agents before they were technically allowed to do so.
That group would also presumably include the Celtics themselves - unless they truly did have an agreement in place with Kemba Walker in the very first minute they were permitted to have contact with the now-former Charlotte Hornets All-Star:
For his part, NBA commissioner Adam Silver acknowledged in early July that the league could do a better job of enforcing its anti-tampering rules.
"My sense in the room today, when it comes to free agency and the rules around it, we have work to do," Silver said at the Board of Governors press conference.
- With h/t to NBC Sports Boston