Onyango collided with Polokwane City player Mbulelo Wagaba late in the first half of their Absa Premiership match on Tuesday.
He awkwardly fell after Wagaba accidentally elbowed him in the head during a collision in the air.
Sundowns have issued a brief video update of their first choice goalie saying is in a stable condition and "feeling much better".
The 33-year-old goalkeeper will undergo multiple tests following the head injury. The club hasn't issued another update when their first choice goalkeeper will be returning.
Mamelodi Sundowns beat Polokwane City 2-0 at the Peter Mokaba Stadium in Polokwane.
The victory, which came at a cost, secured the Brazilians the second spot on the league standings.