There's growing optimism that Boston Celtics center Kristaps Porzingis could make his return to the Eastern Conference playoffs against the Indiana Pacers as soon as Game 4 on Monday, sources told ESPN.

Porzingis has been making progress from a soleus strain of his right calf, accelerating his on-court ramp up and increasing his fitness to make his conference finals debut possible once the best-of-seven series returns to Indianapolis, sources said.

Porzingis -- out since suffering the injury in Game 4 of the first round against the Miami Heat on April 30 -- missed the entirety of a five-game conference semifinals series victory over the Cleveland Cavaliers, and he missed the opening of these conference finals in Boston on Tuesday night, which the Celtics won 133-128 in overtime.

The Celtics won't rush Porzingis back, and they want to continue to be conservative in his recovery, but sources say he is inching closer to a return.

Porzingis was a major offseason addition to the Celtics, bringing with him an immense impact on both ends of the floor for the best regular-season team in the league. At 7-foot-3, Porzingis averaged 20 points and 7.2 rebounds, and he held opponents to 44.3% shooting in the paint, which was an NBA best among contesting defenders of 500 shots or more, per Second Spectrum.

Porzingis arrived in Boston in a three-team offseason trade with the Washington Wizards and Memphis Grizzlies.