He could have shaken Danny Ainge’s hand last spring, thanked him for four good years in green and moved on to another city and another challenge. The Celtics were coming off a truncated season, a lockout was looming, and exit visas seemed possible for several players. But Ray Allen has been around the league long enough — 15 seasons with another starting on Sunday — to know when he’s comfortably situated.
‘‘We’ve all been on bad teams,’’ said the 36-year-old Allen, who endured half a dozen losing seasons in Milwaukee and Seattle before he arrived here. ‘‘When you understand what it’s like to be on a good team you don’t want to throw that away.’’