Signed as a free agent during the offseason, Saito has spent more time on the DL than on the active roster. He missed the first 93 games of the season with a calf injury and an upper respiratory infection.
After compiling a 6.14 ERA in 10 appearances, he began to experience discomfort in his hamstring.
"He's had a hard time staying healthy all year," D-backs manager Kirk Gibson said.
Zagurski was designated for assignment Aug. 11. While he was waiting to clear waivers, the D-backs called Zagurski Sunday night and told him with Saito hurting they would bring him back to the big leagues, provided he cleared waivers Monday morning.
Zagurski cleared and flew to St. Louis on Monday night.
"It's part of the game," said Zagurski, who has a 6.18 ERA this year. "I wasn't pitching very well, they got some new arms in there, it's kind of how the game works."
In an effort to improve Zagurski's effectiveness, Gibson said he planned on using him differently this time around. Rather than have him pitch full innings, he will be asked instead to get a left-handed hitter out or to turn around a switch-hitter that is weaker from the right side.
"Against certain guys in the lineup, he's probably a better matchup," Gibson said. "We've asked him to do more of that. We'll look for more specialized stuff."