Betts was hit in his hand with a 98 mph fastball on Sunday.
With the series back in Boston, the Celtics will aim to close out the Mavericks and secure their 18th championship in franchise history.
For the first time, HBO's "Hard Knocks" NFL docuseries will follow four teams during its in-season edition.
The Boston Celtics announced Kristaps Porzingis is good to go for Game 5 of the NBA Finals against the Dallas Mavericks after missing Games 3 and 4 with a left leg injury.