A potentially seismic shift hit the Eastern Conference finals less than three hours prior to Tuesday night’s start of Game 1 at FTX Arena, when the Boston Celtics announced that forward Al Horford had entered NBA health-and-safety protocols and would be out for the series opener against the Miami Heat. The designation not only had Horford out for Tuesday’s start of the best-of-seven series, but also, based on the NBA’s required quarantine in such cases, possibly out for Thursday night’s Game 2 at FTX Arena and Saturday’s Game 3 at TD Garden. “We found out about Al about two hours ago,” Celtics coach Ime Udoka said, less than two hours before tipoff. Udoka indicated that upcoming testing results could impact the timing of a Horford return. “He’s feeling fine,” Udoka said, “and we’ll go from there, wait to see results of tests and future tests.” Udoka said he was unaware of any contact-tracing issues with others on the team. It is the third time this season Horford, 35, has entered NBA protocols. In addition, the Celtics announced that a second starter, point guard Marcus Smart, would miss the series opener due to the right mid-foot sprain sustained during Sunday’s Game 7 victory over the visiting Milwaukee Bucks that advanced them to the East finals against the Heat. Smart was listed as questionable earlier in the day.