Onyango has not played since damaging his knee in 2-0 win over Bidvest Wits at Loftus Stadium on February 25, with Wayne Sandilands and Kennedy Mweene getting game-time in his absence.
However, Downs coach Pitso Mosimane says he could have fielded the Ugandan in the 2-1 loss to Kaizer Chiefs on the weekend.
"I could have played Denis but this match [against Chiefs] doesn't decide the title," said Mosimane.
"So I decided to leave him for the other eight games to come this month."
The Brazilians are still without midfielder Sibusiso Vilakazi (knee) and defender Wayne Arendse (ankle) due to their knocks.