Tanabe is expected in Boston on Saturday when the Bruins, who have lost four in a row and are in last place in the Northeast Division, play host to the Buffalo Sabres. The Bruins have given up 73 goals this season, the third most in the Eastern Conference.
"Our team is deep at the center position and we felt we had to take this opportunity to strengthen our defense," Bruins general manager Mike O'Connell said. "David is a good young defensemen with NHL experience. He is a strong skater and he will help our team."
Tanabe, 25, had four assists in 21 games for Phoenix this season. He joined the Coyotes in a June 2003 trade with the Carolina Hurricanes, who made Tanabe their first-round draft pick in 1999.
His career NHL totals are 20 goals and 58 assists for 78 points in 317 career games.
Scatchard, 29, has 111 goals, 121 assists and 861 penalty minutes in 542 games in his eight NHL seasons with Vancouver, Boston and the Islanders.
He had four goals and six assists in 16 games in this, his first with the Bruins after being signed as a free agent. He missed four games earlier this season with a groin injury.