The Boston Celtics were reportedly really upset with the tampering around big man Al Horford, according to Brian Windhorst of ESPN.

Horford signed a four-year, $109 million contract with the Philadelphia 76ers. The All-Star big man declined his $30.1 million player option with the Celtics.

The Celtics weren’t reportedly thrilled with how Horford’s free agency played out.

“The Celtics were, from what I am told, one of the teams that kind of stomped their feet with what they thought was tampering,” Windhorst said on the Hoop Collective Podcast. “Not with Kyrie (Irving), with Horford. What happened with Horford, from what I am told, really upset the Celtics.

“They were thinking they were going to be able to negotiate with him, talk to him about a new contract and all of a sudden it was like, he already knew what his market was and what was out there.”

Boston lost Al Horford to the Sixers and Kyrie Irving to the Brooklyn Nets, two teams in their division. President of basketball operations Danny Ainge had an idea Irving was on his way out of town, but the Horford move came as a shock to multiple people in the organization.

The Celtics can't complain too much. They pretty much had an agreement with Kemba Walker before the free agency period officially started.

Tampering happens all the time in the NBA.