While it might be too reductionist to say the Cavaliers really lost Game 1 because of some poor shooting variance, it’s also partly true.
Per Cavs analyst Mike Zavagno, the Cavs shot 16-of-53 (30.2%) on uncontested shots. Meanwhile, the Celtics went 25-of-39 (64.1%) on contested ones. Per NBA Stats, the Cavs went 4-of-20 (20.0%) on 3-pointers in which the shooter didn’t dribble — catch-and-shoots, essentially.
For reference, the Cavs were fifth in the league during the regular season on catch-and-shoot 3-pointers, making 9.1 per game on 38.8% shooting. They averaged 7.3 per game on 37.9% shooting in the three regular-season games against the Celtics. Again, it’s reductionist to say the Cavs just missed shots … but they did miss a lot of open shots.