Keuchel originally landed on the DL in May due to a pinched nerve in his neck, according to McTaggart, who added the lefty still has neck discomfort. He will miss his next start, and Astros general manager Jeff Lunhow said there is no timetable for his return.
The 29-year-old has bounced back this season after a difficult 2016 campaign with a 9-0 record, an MLB-best 1.67 ERA, 0.87 WHIP and 69 strikeouts in 75.2 innings.
Keuchel went just 9-12 with a 4.55 ERA and 1.29 WHIP last season, as shoulder pain hampered him throughout the year.
The 2015 AL Cy Young Award winner has been dynamic when healthy this season, but he hasn't been able to avoid the injury bug.
While Keuchel was able to return from his first DL stint, it's cause for concern that he's still ailing, especially due to his importance at the top of the starting rotation.
Houston has other quality arms such as Lance McCullers Jr. and Charlie Morton, but Keuchel is the unquestioned No. 1. Being without him for an extended period would be a major blow to the Astros.
Keuchel won 20 games, posted a 2.48 ERA and was named to his first All-Star Game in 2015. His performance in 2017 has been on that level, which has helped Houston race out to an MLB-best 42-18 record. A long-term absence would present a major obstacle, although the Astros do boast one of the deepest and most talented lineups in baseball.
Houston is capable of winning slugfests when it needs to, and its 12-game lead in the AL West means the team can take a cautious approach with its ace.