The C's are pursuing Jason Terry, and have also shown interest in Nick Young, according to Yahoo! Sports' Adrian Wojnarowski.
Terry has been with the Mavericks since 2004 but is currently a free agent. Known for his perimeter play, the 34-year-old has long been one of the league's best sixth men, and he would provide the Celtics with some scoring punch if Allen were to leave.
Terry averaged 15.1 points and 3.6 assists in 31.7 minutes per game last season, his 13th in the NBA. He was voted the NBA's Sixth Man of the Year in 2008-09, and remains a solid bench player.
Young, meanwhile, is a younger alternative, having just turned 27. He averaged 9.7 points in 23.5 minutes per contest last season, and was dealt to the Clippers from the Wizards in March. He is also a free agent.
The Celtics have plenty of interest in bringing back Allen, so we may never see Terry or Young in Green, but it's better to be safe than sorry.