RD Rosen (Richard Rosen) wrote mysteries about a detective who played for a fake Providence MLB team. In the second book, however, I'm pretty sure he investigates a mystery related to the Celtics.
Alison Gordon wrote a series about a baseball writer from Toronto. She called the team the Titans, I think, but she was a sportswriter and followed the Jays. The character's name is Kate Henry.
Troy Soos wrote the "Mickey Rawlings" series of baseball novels in the mid-1990s. Rawlings, a fictional up-and-coming baseball player living in the early part of the 20th century, is known as a defender of human rights. The first book in the series is MURDER AT FENWAY PARK, followed by MURDER AT EBBETS FIELD, MURDER AT WRIGLEY FIELD and three other entries. The books feature real major league ball clubs.
Harlan Coben writes a series around Myron Bolitar, a sports agent, who deals with a different athlete in each book, sometimes basketball or baseball players. But I believe the teams are fictional, and usually somewhat peripheral to the story. Enjoyable reading, however.
Myron is more than an agent, of course. He was a star basketball player for Duke, first-round draft choice who busted his knee in his first pre-season game. Each book I've read in the series (all fast, fun, and thrilling) takes the reader inside the world of highly paid pro athletes. The one I'm reading now, FADE AWAY, won an Edgar, and takes us inside the NJ Nets. He calls them the Dragons, but it's recognizable.