Happ, one of the National League's top rookies this season, is 10-4 with a 2.77 ERA. He suffered the muscle injury Saturday during a batting-practice session.
Rather than risk further injury, Happ was scratched, with Jamie Moyer getting the start. Moyer, 12-9 with 5.07 ERA, had started 22 games this season before moving into the bullpen Aug. 10.
"We take BP enough, and I swing enough that this is a freak thing," Happ said. "I have no idea how it happened. I do a lot of stretching and those types of things. [I was doing] nothing out of the ordinary."
"[On Sunday] we were unsure [about skipping Monday's start]," Happ added. "I was just hoping that I would super-heal, but it wasn't quite enough. ... It's concerning for sure, because I was kind of hoping that it was going to be one of those things where you feel something, but then you go to bed and it's gone."
But the pain was still there Monday, even though there was some encouragement to be taken from the fact that the muscle felt better than it did the day before.
"I think we're on the right path," Happ said. "I think that as far as some of the strength tests and examinations that they do each day, today was better than [Sunday]. Hopefully that continues."
Happ, who said he suffered a similar injury in 2005, at first wasn't fully aware the severity of the injury.
"It was sore, and then it's so close from the end of BP to game time on the road that I thought it might have just been a cramp or something," Happ said. "But when the game came around I started cheering and yelling, and I felt something, so I thought that maybe we should ice this.
"I've had this before, and I think it's one of those things that may have a way of being unpredictable and lingering. So we're trying to take the right approach here and trying to knock it out before it gets worse."
Happ wanted to wait to see how he felt Monday morning before making a decision on the start. Happ also hopes to make his next start, but couldn't definitively say when he would be able to again take the mound.
"I'm hoping to [make my next start]. I'd like to," Happ said. "This wasn't easy to decide. I didn't want to come to this conclusion, but it's the smart thing, that's for sure. I'd be really frustrated if I tried to go today and all of a sudden it's six or seven starts [missed] instead of just one or two or whatever.
"It's just a frustrating situation."
Michael Murphy is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.