A left oblique strain has sidelined the right-hander since May 7, dealing the Reds' bullpen a major blow, as Masset hadn't allowed a run in any of his previous six appearances and has only surrendered one earned run all season.
Manager Dusty Baker and the front office evaluated Masset's status on Sunday and opted against the disabled list, for now.
With upcoming series against Philadelphia and Cleveland, and a bullpen taxed by Saturday's 16-inning contest, there may be no choice but to make a move.
"It's going to be at least 5-7 days before he can pitch again, so we can't go against Philly and Cleveland short-handed," Baker said. "[Those teams] are so left-handed dominant, we probably need another left-hander."
Adding to the frustration, Masset isn't even sure how the injury occurred.
"It's just one of those freak things," Masset said. "It's very disappointing. I was throwing good, I was in a really good groove. I'm just trying to mentally stay strong throughout the whole thing and just keep myself in the game and I'm here for my team and rooting everybody on. I know my bullpen is going to pick me up."
Both Baker and Masset indicated that the injury had been improving as Masset tested it each day by playing long-toss. Nonetheless, his unavailable status remained the same Sunday.
"It felt really good overall [Saturday]," Masset said. "It's really small, but we're just taking it day-to-day. It's just tossing and getting loose and getting back into the swing of things, but it's been going really good, so hopefully the ETA will be a lot sooner than we anticipated."
Not having Masset available may have hampered the Reds on Friday and Saturday in San Diego.
In a situation when Baker normally would've summoned the right-hander -- with the bases-loaded, one out and the score tied at 3-3 -- the manager instead went to veteran David Weathers, who promptly served up a single to Scott Hairston for the game's decisive runs.
"[Masset] sure was throwing good," Baker said. "[Friday] night, I know that was normally his time, but I only had two guys I could use to get a ground ball for a double play. We got the ground ball from [Scott] Hairston, but it just went through the hole."
Saturday's lengthy game required Micah Owings, normally the fifth starter, to throw 5 2/3 innings in relief, complicating matters on Wednesday when Owings was scheduled to make his next start.
"We're going to have to change something, but we don't want to change our rotation," Baker said. "We like our rotation, so I'll have to get together with [pitching coach Dick Pole]. We're working on it right now."
Tim Powers is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.