Myron Stovell, 39, was shot around 12:30 p.m. Saturday while conducting a practice for children ages 7-15, WCBV-TV, Boston, reported.
Team spokeswoman Jenn Reed said there had been an altercation between Stovell and the boy, whom police said was a known gang member.
"He hit the kid's dog, that's why they did it to him," Reed told the Boston Globe.
Stovell was shot once in the leg, Police said the injury wasn't life threatening.
Boston Police Commissioner Edward Davis ordered an increase in police patrols in the area, and called for community support.
"The victim in this incident is a role model in our neighborhoods," Davis told reporters. "The individuals responsible for this violent act should be scorned by their families and held in public contempt by the community."