At Safeco Field on Aug, 21, the most improbable streak of this baseball season will end.

The Astros will play the Mariners at 9:15 p.m and, for the first time this year, deploy a starting pitcher who is not among the five-man rotation with which they began the season.

It could happen before then, certainly, if another injury affects the four Astros starters who remain healthy.

But if they remain unharmed and the status quo sticks, Aug. 21 is the first game when a fifth starter would be required in the wake of Lance McCullers Jr.'s injury. Upcoming off days line up conveniently, allowing Justin Verlander, Dallas Keuchel, Charlie Morton and Gerrit Cole to comprise an adequately rested four-man rotation until the Astros' next road trip.

Only two major league teams — the 1966 Dodgers and 2003 Mariners — have ever completed a season using five starting pitchers. The Astros required 11 during their World Series run last season. In manager A.J. Hinch's first season, they used 13 en route to a playoff appearance.

None of his five starters this season has missed a turn in the rotation. It's an astounding feat, due in equal part to proficiency and luck.

Even the circumstance that will end the streak — McCullers' forearm muscle strain — is fortuitous. When McCullers exited Saturday's 14-0 win over the Dodgers favoring his right elbow, fear of a damaged ulnar collateral ligament or Tommy John surgery filled the organization.

They "dodged a bullet," as general manager Jeff Luhnow termed it, but a rotation spot remains open. McCullers will likely not pitch again until September. Whether he returns to the rotation or goes into the bullpen solely depends on how much time the club has to build his arm back up.

While the team needs a fifth starter, options abound.

Three exist on the current 25-man roster. Collin McHugh and Brad Peacock are proven major league starters. Cionel Perez started 27 minor league games but is a veteran of only six major league innings — all from the bullpen.

McHugh, a 102-game starter in his first four seasons with the club, is outwardly the most logical option. If not for the offseason acquisition of Cole, McHugh would be a member of the starting rotation.

Instead, the former 19-game winner was shipped to the bullpen, where he has surprisingly carved a crucial role.

Five of McHugh's last 12 outings ended in "holds." Hinch has leaned on him for the seventh or eighth inning of a game in doubt — the bridge to closer Hector Rondon.

McHugh's 1.00 ERA is second among all major league relievers who've thrown at least 50 innings. He has yielded two earned runs in his last 16⅓ innings.

"McHugh can do a lot of good things for us, and we have to determine where those innings come from," Hinch said. "Certainly, if you take him out of the bullpen and put him in the rotation, you're gaining something in the rotation, and you're taking something out of the bullpen."

Adding Ryan Pressly and Roberto Osuna at the non-waiver trade deadline could mitigate McHugh's absence. Both were acquired for the sole purpose of pitching high-leverage innings. Hinch said Monday the two could "soften the blow" should McHugh enter the rotation.

"But I also like the look of having multiple (leverage) options every day," Hinch acknowledged.

Hinch and Luhnow both named four righthanded minor league candidates for a start — Josh James, Brady Rodgers, Trent Thornton and Rogelio Armenteros.

Rodgers is the only candidate currently on the 40-man roster. After Buddy Boshers was claimed off waivers by the Pirates on Tuesday, the Astros have two open spots on the 40-man roster.

Armenteros had an eye-opening stint in major league spring training. James is enjoying a resurgent season, his fourth full one with the organization, having posted 154 strikeouts in 99 innings between Class AA Corpus Christi and Class AAA Fresno.

Luhnow also mentioned Framber Valdez — who had a 0.40 ERA in July with Class AA Corpus Christi. Monday, he was named the organization's Minor League Pitcher of the Month. Thursday, the lefthander was promoted to Class AAA Fresno.

MLBPipeline ranks Valdez the organization's No. 11 prospect. Armenteros is one spot behind.

After this season, Valdez, James, Thornton and Armenteros are eligible to be selected in December's Rule 5 draft. Adding them to the 40-man roster, whether it be now in a spot-start situation or after the season, protects them from selection and keeps them within the Astros organization.

"All of those guys are worthy of consideration," Luhnow said Monday. "We'll sift through it, and we'll figure out how many starts it's likely to be, (and) is that player likely to be added to the roster anyway this offseason or this September? And if so, is it a little easier to do that now?"

Bear in mind, too, that rosters expand in September.

"Trying to get to September is key," Hinch said. "There's only a couple starts before September, when we will have as many pitchers as we want to take the innings."