CLEVELAND—Darius Garland made two free throws with 9.4 seconds left and the Cleveland Cavaliers rallied from 19 points down in the third quarter to stun the Boston Celtics 91–89 on Saturday night in the opener of a two-game set. Dennis Schroder had an opportunity to force overtime for Boston, but missed a jumper from the right baseline as time expired. There were five lead changes and seven ties in the final seven minutes. “There is no basketball reason why we should have won, but there was a collective spirit,” Cavaliers coach J.B. Bickerstaff said. “Momentum is a hell of a thing. When you start to ride that wave in either direction, it changes the game.” The teams will meet again Monday at Rocket Mortgage FieldHouse. The Celtics built their largest lead at 72–53 with 3:05 left in the third, but the Cavaliers answered with a 24–4 run to go ahead …