Downs last week announced the signing of Nascimento from Portuguese side Academia on a three-year deal, to bolster their defence following the exit of Ramahlwe Mphahlele, who has since joined Soweto giants Kaizer Chiefs.
The reigning Absa Premiership champions face a hectic schedule as they continue in their CAF Champions League campaign, with the 2016/17 domestic season also on the horizon.
Mosimane explains that the 29-year-old, with all his experience, has been brought in as cover for the ageing legs of Thabo Nthethe and Wayne Arendse as they face a prolonged fixture schedule.
"Ricardo is an experienced defender, I went to watch him in Portugal and I followed him for a year," said Mosimane.
"He's a typical Thabo Nthethe, but with a left foot, because we want those diagonals [passes] - if you can check our build-up play from left to right.
"Also, at Thabo and Wayne's ages they can never play football for two years, so we have to rotate them."