Roach, 25, who the Cubs claimed off waivers in November from the Padres, was 7-1 with a 2.29 ERA in 14 starts at Triple-A Iowa. He appeared in 16 big league games and made one start last year with San Diego, compiling a 4.75 ERA.
The right-hander took a circuitous route Friday to get to Busch Stadium, flying from Des Moines to Dallas to St. Louis.
"I had to go backwards to go forwards," Roach said.
There are no "alarms going off" regarding Wada's shoulder, manager Joe Maddon said. The lefty had to leave his last start after two innings because of the cramping. He has not undergone an MRI. The Cubs say there's nothing structurally wrong. Wada did play catch on Friday at Busch Stadium.
"The thing about Wada is that we're still not 100-percent sure where that is right now -- I don't know how bad he is," Maddon said. "He's doing better. It's not awful. We're just concerned that it's not progressing to the point where we have a definite answer."
The Cubs did consider right-hander Edwin Jackson and lefty Travis Wood, who are both in the Cubs' bullpen, but Jackson was needed Thursday and Maddon says Wood is "really valuable" in a relief role.
A corresponding roster move will be made Saturday, and it would most likely be a pitcher, Maddon said.
"We really can't lose any more bench players right now," Maddon said.
Roach will have a tough assignment, having to face the Cardinals, who boast the best record in baseball.
"That's why I play the game -- it's fun," Roach said.
Because he has pitched in the big leagues, he's not as nervous.
"There's less of the butterfly effect -- I still have some, it's still the big leagues -- but this isn't my first big league start or big league appearance," Roach said.
He's not looking too far ahead. All the Cubs have told him is that he's starting on Saturday.
"And they said, 'good luck,'" Roach said.