Aric Almirola subbed for Scott Speed in Happy Hour at New Hampshire Motor Speedway on Saturday.
According to Red Bull General Manager Jay Frye, Speed was suffering from flu-like symptoms and went to the infield care center for treatment.
"We'll see how Scott does tonight and go from there," Frye said. "We hope it's a 24-hour bug but we've asked Aric to stay overnight as a precautionary measure."
Speed qualified 28th for Sunday's Sylvania 300. He was posted as 33rd fastest in morning practice with a time of 128.576 mph.