A game shall be forfeited to the opposing team when a team is unable or refuses to place nine players on the field.
A game is forfeited when the short-handed team:
(1) cannot start the game with 9 players;
(2) on defense, cannot (or will not) place 9 players on the field; or
(3) on offense, reaches a vacant spot in the batting order. [BRD]
The language in this rule is mandatory, not discretionary: "the game shall be forfeited." Unlike amateur codes, there is no option in the OBR to play with eight players.
There has never been a MLB game forfeited because the team was unable to field nine players, but there have been games forfeited when the team refused to field nine players. See http://www.retrosheet.org/forfeits.htm