The 42-year-old made his season debut hours after the roster move, pitching one inning against the Astros in the D-backs' 12-3 win and allowing an unearned run on a hit and a walk.
With the appearance, he became the first player of Japanese descent to pitch for the D-backs.
Saito had missed the entire season up to Saturday with calf and shoulder injuries.
"He's had some setbacks, but he's confident and feels ready to go," manager Kirk Gibson said. "We met yesterday, and I tried to get a feeling of how we wanted to proceed. He told me he was 100 percent healthy. His calf is good, his shoulder is good, no sickness, he's ready to dedicate himself to this team."
Before Saturday, Saito most recently threw three scoreless innings for the Arizona League D-backs from July 13-19. He also appeared in four games for Triple-A Reno from June 5-14, allowing three earned runs in 2 2/3 innings.
"He's had plenty of rehab work," Gibson said. "He said he was ready for better competition, so I said, 'Well, there's some pretty good competition here.'"
Moving forward, Gibson plans to use Saito in the sixth or seventh inning, but he won't hesitate to throw him later in games because of his experience.
The D-backs will, however, try to limit his workload in order to ease him into a groove.
"He's going to be a little rusty, probably. I won't be able to throw in him back-to-back days," Gibson said. "Maybe an inning one day and a third of an inning the next day, but I wouldn't say two innings."
In 323 career relief appearances, he is 21-15 with 84 saves, a 2.17 ERA and 389 strikeouts.
"Every year he's pitched, he's had good results. We're counting [on him to be] ready to pitch and navigate through a lineup," Gibson said. "We saw Saito as somebody who has a ton of experience and knows how to pitch in leverage situations. That's an advantage for us."