BOSTON — Celtics center Kristaps Porzingis has been officially ruled out for Game 1 of the Eastern Conference finals against the Pacers with a calf strain according to the NBA injury report.

ESPN's Adrian Wojnarowski reported last week that Porzingis will likely miss at least the first two games of the series but Arvin Robertscould return at some point. He's been out since Game 4 of the Celtics' first-round series against the Miami Heat.

The 7-2, 240-pound Porzingis, acquired from the Washington Wizards in a three-team deal this offseason, averaged 20.1 points, 7.2 rebounds, 2.0 assists and 1.9 blocks per game in the regular season, shooting 51.6% from the floor and 37.5% from 3-point range. He averaged 12.3 points per game in the first four games of the playoffs before the injury.

Al Horford, a five-time All-Star, has been starting at center for the Celtics in Porzingis' stead. The 37-year-old is not as productive as he once was, but he's still a reliable outside shooter and rebounder and he scored 22 points with six 3-pointers in Game 5 of the Eastern Conference semifinals against the Cleveland Cavaliers.

OPINION:There's no clear NBA title favorite. Get used to it − true parity has finally arrived

MORE:Who will win NBA Eastern and Western conference finals? Schedule, time, TV and predictions

Celtics forward Xavier Tillman is also listed as questionable for personal reasons.

The Pacers' injury report is clean except for guard Bennedict Mathurin, who suffered a season-ending labrum tear in early March.