Austin faces two months or more on the sidelines after suffering a suspected hamstring tear.
The striker pulled up in last weekend's 1-1 Premier League draw with Huddersfield Town and is now set for a prolonged absence.
Austin headed in the opener against Huddersfield but limped off with 11 minutes to go.
"Charlie, I think he will be out more than normal ... this evening he will have another test to analyse a little bit better his injury, the grade of his injury," Saints boss Mauricio Pellegrino said.
"We will see but I expect Charlie will be out for a couple of months, minimum. It is a big pity because Charlie was in a really good moment."