That makes right-hander Stephen Fife, who made an impressive cameo start July 17 and is at Triple-A Albuquerque, the logical choice among Minor League starters on the 40-man roster.
Fife allowed one run on four hits in six innings of a no-decision against the Phillies.
"We have some options," Mattingly said. "He will come out of the system somewhere. Who it is is up in the air."
Mattingly rejected the idea of starting long reliever and former starter Jamey Wright.
"I don't like what that does to the bullpen," he said. "I don't want to get us caught in that spot."