The BMC Racing Team rider will still start the race, the team's Twitter account confirmed on Thursday, and a team source separately confirmed to Cyclingnews.
Porte's illness, team doctor Daniele Zaccaria said on the BMC account, began when he woke up with some "intestinal issues this [Thursday] morning.
"We have been monitoring him during the day and although it is nothing too serious, we feel the best thing for his recovery is to rest for the remainder of the day."
Porte was expected to recommence light training on Friday and then be on the start line on Saturday, the team added. Although "not ideal," the team said, they were confident that rest would help Porte get back to his pre-race form.
The Australian opted for the Vuelta after a bad crash in stage 9 of the Tour de France wrecked his chances in his initial main Grand Tour target of the season.
Porte was already in the news on Thursday, too, after a long-rumoured confirmation that he would be riding for Trek-Segafredo in 2019 finally came through.