Team Sky, who have won the last four Grand Tours, said last week that Thomas and Froome would skip the event to participate in next month's Tour of Britain.
"In Michal Kwiatkowski, I believe we have a rider who is capable of one day winning a Grand Tour," team principal Dave Brailsford said in a statement.
The 28-year-old Kwiatkowski is participating in his second Grand Tour of the same season following the Tour de France.
"He has already shown his ability in one-day races and now he is progressing well in his development as a stage racer," Brailsford added.
"David de la Cruz is in a similar situation and comes into the race in great form and has shown signs of progression in his first season with us."
SBS will broadcast all stages of the 2018 Vuelta a España.
Team Sky's squad for the race in Malaga also included Tao Geoghegan Hart, Jonathan Castroviejo, Sergio Henao, Salvatore Puccio, Pavel Sivakov and Dylan van Baarle.
Kwiatkowski said his selection for the Vuelta was another important step in his career.
"I've never raced two Grand Tours in one season, so I'm intrigued to see how my body will react to that challenge. But I don't feel pressure, just excitement before a new experience," Kwiatkowski said.
De la Cruz added: "It's so exciting. The Vuelta has been my big goal of the season. I've worked really hard in the months following the Giro d'Italia and finally, it's time."