The breakup doesn't really hit you until you see your ex with someone else.

Such is the case for Boston Celtics fans and Al Horford.

The free-agent big man officially signed with the Philadelphia 76ers on Wednesday, meaning the Sixers could officially tweet a photo of Horford in a No. 42 Sixers uniform that may make C's fans sick to their stomachs.

We liked him better in green and white.

Horford has been gracious on the way out, thanking Boston and its fans in a farewell note Wednesday on social media. He also was one of the Celtics' most respected players during his three-year tenure with the club.

But now the 33-year-old will be competing against the C's four times a year on the rival Sixers, a reality that Philly drove home Wednesday by announcing his signing.

"Al’s playmaking, elite defensive talents and veteran leadership confirm why he is a crucial addition to our roster," Sixers general manager Elton Brand said Wednesday in a statement. "We have gained a championship-level teammate that will not only complement our current makeup, but will help grow our young core as we strive for the highest level of success.”

Click here to download the new MyTeams App by NBC Sports! Receive comprehensive coverage of your teams and stream the Celtics easily on your device.