Houston general manager Jeff Luhnow said the team is waiting to find out exactly what is ailing Weiland, who has been on the disabled list since Friday after making three starts. An MRI performed last week showed Weiland's shoulder was structurally sound, but the pain lingered.
"We're not even sure it was an infection, but whatever it is, he had to have his shoulder cleaned out and they're still diagnosing it," Luhnow said. "The culture is still being analyzed."
Luhnow was initially hopeful Weiland would be able to return to the rotation after the minimum of 15 days, but that's not going to be the case.
"If he has to be treated for an infection, he has to go through an antibiotic course, and we just don't know how long that will take and how much he can do during that time and how long it will be before he's ready to throw in games," Luhnow said. "We'll proceed with Plan B, which is to figure out another way to get a starter into our rotation."
Because of Thursday's off-day, the Astros can get by with a four-man rotation until May 8, when they will have to call up someone to make a start. The spot can't be filled by Jordan Lyles, who made a spot start Sunday and was sent back down. He has to stay in the Minor Leagues for 10 days before he can get recalled.
Among the candidates to start at Triple-A are right-handers Henry Sosa and Aneury Rodriguez, both of whom pitched in the Majors last year.
"We're definitely looking at, there's no doubt, and throwing around names and possibilities," manager Brad Mills said. "It's too early yet."
Weiland, acquired from the Red Sox along with shortstop Jed Lowrie in a trade in December, is 0-3 with a 6.62 ERA in three starts this year.