Watch: 14-year-old becomes youngest player to debut in MLS history

At 14 years, 293 days old, Sullivan is also the youngest player to compete in any of the major professional North American sports leagues, including the NFL, NBA, MLB and NHL. That breaks the record previously set 20 years ago by Freddy Adu, who, oddly enough, also suited up for the Union from 2011 to 2012.

Fittingly, Sullivan was on the field to watch his older brother, Quinn Sullivan, smash one into the back of the cage in a 5-1 Union victory over the New England Revolution.

In May, Sullivan, a Philadelphia native, agreed to a first-of-its-kind contract with the Union. The deal includes an agreement that the franchise will send him to Manchester City of the English Premier League once he turns 18.

"The collaboration between the Union and the City Group was — I think that did it for me," Sullivan said. "I always watch Man City. They're like every kid's dream team. For [Philadelphia and Man City] to come together and agree on something — I sat with my family and my agents and we decided that it was the best plan."