The Westgate Las Vegas SuperBook installed the Cavaliers as a minus-275 favorite over the Celtics, who have home-court advantage in the best-of-seven series.
The line for Sunday's Game 1 in Boston opened at pick 'em at the Westgate.
LeBron James has faced the Celtics six times in the playoffs, winning the past four series and six straight postseason road games in Boston. James is averaging 34.3 points per game this postseason, the second-highest scoring mark of his illustrious playoff career.
The Cavaliers won two of three meetings with the Celtics in the regular season, including a 121-99 rout in Boston on Feb. 11.
Now the Celtics get another shot at James and the Cavaliers, who beat them 4-1 in last year's Eastern finals.
"Obviously guys have been going down all year. It's like you never know who's going to go down, things like that, but we find a way," Celtics point guard Terry Rozier told reporters Wednesday night. "We pull together. I'm not saying we're better or not [than last year's team], but we're definitely going back to the Eastern Conference finals, and that's a blessing."
The Cavaliers also were an underdog in the second round, but they swept the top-seeded Toronto Raptors to reach the conference finals for a fourth straight season. James is looking to reach the NBA Finals for an eighth straight year.
The winner of the Eastern Conference is expected to be a significant underdog against the Western Conference champion in the Finals. The Golden State Warriors are the title favorites at 5-8, followed by the Houston Rockets at 12-5. The Cavaliers are 11-2, with the Celtics at 18-1.