This is after the passing of his father in Uganda, and according to the club, Onyango will leave immediately to be with his family.
"It is with great sadness that we learnt of the passing on of the father of Denis Onyango," reads a statement on the Sundowns website.
"The club has released Denis to mourn this great loss and he will leave immediately for Uganda with his family.
"He has the full support of all his teammates, technical team, management, the board and the club president. We all wish him and his family all the strength during these trying times," concluded the statement.
Onyango's loss comes a few days after assistant coach Manqoba Mngqithi lost a family member and missed the encounter against AmaZulu last Sunday.
In Onyango's absence, Pitso Mosimane will either bring in Kennedy Mweene or Reyaad Pieterse for the match against the Lions of the North.
The two goalkeepers have played in recent weeks, with Mweene last featuring in Sundowns' 11-1 win over Cote d'Or in the Caf Champions League.
Pieterse was in goal in the first 45 minutes of Sundowns' 4-2 loss to Kaizer Chiefs in the Shell Helix Cup two weeks ago.