The striker started his assessment last week with Sundowns.
Mosimane says Nathan Junior first surfaced on his radar when he recruited the forward's compatriot Ricardo Nascimento in 2016 during his loan spell with Portuguese outfit Coimbra.
"We are watching him," said Mosimane, whose side were held to a 1-1 draw by log leaders Bidvest Wits at Lucas Moripe Stadium on Sunday.
"We are looking at him. We will not make a decision now. We are not really under pressure to make a decision. We will look at him. He is a good player.
"I saw him when we recruited Ricardo. I went to Portugal to watch Ricardo and he was playing against Ricardo. He did well."
Downs also have South Americans Gaston Sirino and Jose Ali Meza of Uruguay and Venezuela respectively on their books.