Another one of the Celtics' top in-house free agents appears set to not only opt out of his deal with the team but also move along from Boston, too.

Though it was reported earlier Tuesday that Horford was expected to decline his $30.1 million player option for 2019-20, it was tinged toward a potential return. Steve Bulpett of the Boston Herald reports that contract talks have now apparently shifted toward Horford exiting Boston. News of Horford's imminent departure from the Celtics was also reported by ESPN's Adrian Wojnarowski as well.

Less than a week ago, Celtics star guard Kyrie Irving decided to not opt into his deal for next season. That leaves the Celtics reportedly preparing for a nightmare scenario in which both Horford and Irving walk in free agency, with nothing to show for their recent run of high-profile asset acquisition.

Bulpett reports that Horford is expected to sign a four-year contract somewhere else this summer. Horford's side was interested in staying in Boston at a more team-friendly contract number if the team were willing to add two additional years to the deal, Bulpett reports. Boston seemed willing to do that and the two sides engaged in talks along those lines, but then things fizzled out suddenly.

Wojnarowski reports there was "too great of a gulf" between what the team is willing to offer and what Horford wanted in a long-term deal. Additionally, Wojnarowski reports Celtics president Danny Ainge and Horford's agent, Jason Glushon, aren't expected to restart contract talks.

According to Kevin O'Connor of The Ringer, the Dallas Mavericks are "considered favorites" to land Horford.

A. Sherrod Blakely of NBC Sports Boston reports the Nets might also be in the running for Horford's services, too.

A team to keep an eye on in the Al Horford sweepstakes is the Brooklyn Nets, league source says. The #Nets are a team on the rise, Al's close ties to head coach Kenny Atkinson and the team's cap space mentioned as the main reasons. — A. Sherrod Blakely (@ASherrodblakely) June 19, 2019

Wojnarowski previously reported Horford and the Celtics had interest in working toward a new deal in July, one that would help Boston's salary cap flexibility. Team president Danny Ainge said in early June that he was hoping to discuss restructuring the All-Star big man's contract, a move he called a priority this summer.

A five-time All-Star, Horford averaged 13.6 points, 6.7 rebounds, 4.2 assists and 1.3 blocks per game last season for the Celtics, appearing and starting in 68 games. Overall, the former No. 3 pick in the 2007 draft has averaged 14.1 ppg, 8.4 rpg, 3.2 apg and 1.2 bpg in his career.

With Horford seemingly on the way out -- and Irving likely to move on as well -- the Celtics could have salary cap space in the high $20 millions.