Philadelphia Union interim manager Ryan Richter remembers it well.

He was mindful of midfielder Cavan Sullivan 's mood earlier this season. Sullivan was finding playing time hard to come by for a side that was struggling mightily. Even as Richter was spending most of his time managing the Union's reserve team, he couldn't help but notice the demeanor of a player he first coached as an 8-year-old.

"He was pissed," Richter said of Sullivan in an interview with ESPN. "He wanted a chance to play."

He eventually got it, and he has been taking out that anger on the rest of MLS ever since.

As Sullivan and the Union head into this weekend's game against FC Cincinnati , the 17-year-old is nursing an ankle injury, but his rise has been the story of the MLS season. His 21 goal contributions have already broken the league record for a teenager, eclipsing the 20 recorded by the New England Revolution 's Diego Fagundez in 2013.…