Hendriks has posted a 6.87 ERA in eight starts this year, while De Vries has a 7.88 ERA in two relief appearances totaling eight innings.
Gardenhire said he'll meet with the pitchers and pitching coach Rick Anderson to make a decision on who will make the start in Oakland.
"We have a few of days to figure it out," Gardenhire said. "We used two pitchers last night and I guarantee you one of those pitchers will make that start. It's a 50/50. We'll let Andy sit down and talk to both pitchers and figure out the way to go."
Rhett Bollinger is a reporter for MLB.com. Read his blog, Bollinger Beat, and follow him on Twitter @RhettBollinger. This story was not subject to the approval of Major League Baseball or its clubs.