Trying to bounce back from a brief stay in his last outing, Masterson looks to help the visiting Cardinals avoid their first three-game sweep at the hands of the Miami Marlins in 18 years Wednesday night.
In two starts since being acquired from Cleveland on July 30, Masterson (1-1, 11.25 ERA) has allowed five runs, seven hits and walked three in each. He lasted six innings during a 9-7 win over Milwaukee on Aug. 2, but two-plus innings in Friday's 12-2 defeat at Baltimore.
That marked the third time this season Masterson went just two innings.
"This is not the best thing you want to do when you come to a new team," said Masterson, who went 4-6 with a 5.51 ERA in 19 starts for the Indians.
"It's really just a lot of minor things. It just needs to click that little bit. A lack of location and the sinker flattening out."
Until now, Matheny and his coaching staff opted not to tinker with Masterson's approach or mechanics until seeing him pitch in person.
"We've been in that mode of just gathering information because there is nothing worse for a guy than for us to come in and say, 'OK, scrap all that stuff you've been working on.'" Matheny told MLB's official website.
"Right now, he's trying to get it in the right place."
In Masterson's only start against the Marlins (59-60) in 2012, he allowed two runs in seven innings while not factoring in the decision of a 3-2 defeat.
Miami has not swept a series of at least three games from the Cardinals (62-56) since Aug. 27-29, 1996 on the road - a season after last doing so at home.
While Miami won for the fourth time in five games, St. Louis dropped to 1-4 on a six-game trip.
A night after recording four hits in Monday's 6-5 loss, Matt Holliday went 0 for 4. He's 1 for 6 against Miami's Nathan Eovaldi (6-6, 3.99).
The right-hander went 0-2 with a 9.92 ERA in his first three starts following the All-Star break before allowing an earned run over 15 innings in his last two outings - both against Cincinnati. He yielded five hits over eight scoreless innings in Friday's 2-1 road victory.
"Same approach: Keep attacking," he said.
Though Eovaldi allowed a run and two hits over seven-plus innings of a 2-1, 10-inning victory over the Reds on Aug. 2, he is still is 0-2 with a 7.94 ERA in his last six home starts.
He'll face the Cardinals for the first time in Miami, and is 0-1 with a 3.57 ERA in three starts at St. Louis. He gave up three runs in six innings of a 3-2 loss July 4.