Sainz won the Dakar Rally in a VW Race Touareg in 2010 and remains part of the VW setup as it moves into the WRC. Nissen said it was the Spaniard's professionalism which helped establish VW as such a dominant force on the Dakar, where it took three back-to-back victories.
Nissen said: "We have seen on the Dakar, we went fast forwards when we got Carlos Sainz in the car with his extremely professional attitude.
"It was not because the others were bad drivers, but the other drivers came from the classic Dakar, whereas Carlos came from the motorsport where you need to be all the time fast, all the time committed, everything must be 100 per cent, everything.
"We tried to give that to Carlos and we came a long way. In Carlos, we had the push. I am very happy to have Carlos on my side now, he can help me push."
Sainz has already driven the Polo R WRC development car - and has been asked to compete in the World Rally Championship in the machine by Nissen.
"Carlos is here now because he has a long-term agreement with Volkswagen," Nissen added. "[To] build up the team and to follow the team when we get started. His experience is invaluable to us."
Sainz has declined the opportunity to drive, but admitted that he will continue to test the Polo in the future.
"It's very nice to be part of the programme here with Volkswagen," said Sainz. "I have good memories of Dakar and I know this team can be strong in the WRC, but we have work to do now. We are new to this side of the sport and we have a lot of testing to do before we can compete with Ford and Citroen."