"We don't know about Henderson," manager Mike Redmond said. "We'll see. His start on Sunday is questionable."
If Alvarez isn't ready, Brad Hand is a candidate to get the nod in the series finale with the Braves at Marlins Park.
The Marlins don't believe Alvarez will miss extended time, but with an oblique injury, they are playing it cautiously.
The rotation also is in limbo with the fifth-starter spot. Brad Penny had a rough outing on Tuesday, giving up four runs in three innings in an 8-6 loss to the Mets.
The team is weighing what to do with Penny's spot, which comes up again on Tuesday at Milwaukee. Tom Koehler is expected to pitch on Monday.
If Penny isn't ready, Alvarez could slide into that day, if he is ready. Top prospect Andrew Heaney, a September callup, is another possibility.
Heaney threw a bullpen session on Wednesday.
Joe Frisaro is a reporter for MLB.com. He writes a blog, called The Fish Pond. Follow him on Twitter @JoeFrisaro. This story was not subject to the approval of Major League Baseball or its clubs.