ESPN and The New York Times reported Walker, the Charlotte Hornets' longest-tenured active player, will sign a four-year, $141 million contract with the Celtics. The veteran point guard averaged 25.6 points, 4.4 rebounds and 5.9 assists last season.
Irving cannot officially sign his new contract until Saturday.
Walker was the No. 9 overall pick in the 2011 NBA Draft from the University of Connecticut. The three-time NBA All-Star averaged 19.8 points and 5.5 assists in 605 career games (560 starts) with the Bobcats/Hornets franchise.
Boston has not made the NBA Finals since 2010.