In an interview with the regional Dutch broadcaster 1 Limburg, Dumoulin insisted team leadership would go to the rider who emerges as the stronger.
Dumoulin, who joined Jumbo-Visma from Team Sunweb for 2020, sacrificed his own chances in order to support Roglič at the Tour de France. Dumoulin was a key lieutenant as the team dominated the race but it ended in disappointment as Roglič was usurped by Tadej Pogačar on the penultimate-day time trial.
Dumoulin again played a support role at the weekend as Roglič won Liège-Bastogne-Liège, and did not rule out the same thing happening at the Vuelta. However the opposite could also happen.
"If I am better in the Vuelta, he will ride for me and if Primož is better again, then we have the same scenario as in the Tour," Dumoulin said.
"I don't want to ride for myself and then finish seventh... if he finishes fourth and I've done no work for him."
Roglič's plans for the rest of the season were unclear in the aftermath of his crushing Tour de France defeat but he bounced back to win Liège and seems motivated to carry on racing and defend his Vuelta crown.
The Slovenian won his first Grand Tour in Spain last year and it was also the scene of Dumoulin's breakthrough, when he came close to winning the race in 2015.
Dumoulin won the 2017 Giro d'Italia and finished second at the Corsa Rosa and the Tour de France in 2018. However his form this season and his prospects at the Tour were hampered by his long spell without racing since he crashed out of the 2019 Giro.
"I am where I want to be," Dumoulin said of his condition as he recovered from the efforts of the Tour de France.
"I'm riding very well and getting a little better every week. I hope to really feel the top form again - if isn't it this year, then next year."
The 2020 Vuelta a España, which has been reduced to 18 stages, kicks off in the Basque Country on October 20 and finishes in Madrid on November 8. There's one individual time trial on stage 13 – a 33.7km course that's largely flat until a short climb at the finish, while summit finishes at the Tourmalet, Angliru and Alto de la Covatilla also feature.
Jumob-Visma are also expected to send Tour de France domestiques Sepp Kuss and Robert Gesink to Spain as part of a strong line-up.
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