"For the Vuelta it's incredibly important that we have Contador here. He's got the most spectacular racing style in the peloton and he's our biggest international name," Guillen told Reuters ahead of Saturday's start.
"His presence will give the race a huge boost in terms of interest for cycling fans."
Contador won the 2010 Tour but had the title taken from him after testing positive for the anabolic agent clenbuterol.
The 29-year-old Spaniard finished fourth on his competitive return in the Aug. 6-12 Eneco Tour in the Netherlands, 55 seconds behind winner Lars Boom.
Guillen believes Contador's main rival for the Vuelta title will be Tour runner-up Chris Froome. The Briton is oozing confidence after his exploits in France last month before he claimed Olympic bronze in the time trial on Aug. 1.
"He's Sky's clear leader here and he won't be making any gifts. He'll be here to win. If he does win he won't just have won a Grand Tour, he'll have beaten one of cycling's greatest riders (Contador)," said Guillen.
Froome finished second by 13 seconds in last year's Vuelta behind Spain's Juan Jose Cobo, but this year there are 10 summit finishes and Froome looked effortless in the mountains during the Tour.
After his recent efforts Olympic time-trial champion and Tour winner Bradley Wiggins is skipping this year's Vuelta, usually staged in the shadow of the Tour de France a month earlier.
"On paper, a route as mountainous as this favours the climbers," said Guillen said, anticipating a Contador v Froome showdown in the hills.
"If we could reach the Bola del Mundo with two riders still battling for the top spot, just like in 2010, that would be a dream finish."
The Bola del Mundo climb, the finale of the penultimate stage of this year's Vuelta, was where Vincenzo Nibali produced a gritty comeback against Ezequiel Mosquera two years ago to seal a first Grand Tour title.
Guillen said there were other sections of the Aug. 18-Sept. 9 race that should provide drama.
"There are some stages that could well catch riders out, like the stage 12 summit finish at the Mirador de Ezaro. It's surprisingly difficult."
This year's Vuelta summit finishes range from the short but very steep Fuerte de Rapitan on stage six to stage 15's daunting ascent to the Covadonga Lakes, home to some of western Europe's last wolves.
After the Bola del Mundo climb just outside Madrid the 3,361-kilometre race ends in the Spanish capital, having started with a 16km team time trial on Saturday that finishes in Pamplona's bullring.
The Vuelta returns to the Basque Country for the second year running for some tough early stages, with a finish on the famous Arrate climb on Monday.
"There was a 33-year gap between the Vuelta's previous visit to the Basque Country and last year's return, which was way too long a gap," said Guillen.
"We wanted to get back there as soon as we could."