Onyango missed the goalless league draw with Baroka on Tuesday and previous 5-1 rout of Leones Vegetarianos in the CAF Champions League after picking up the knock in the Tshwane derby win over SuperSport United at Lucas Moripe Stadium on December 1.
The Ugandan, who has been nominated for CAF's African Player of the Year award along with ex-Sundowns teammate Percy Tau, will make his return in the new year.
The Brazilians will kick off 2019 with an Absa Premiership clash with Soweto giants Kaizer Chiefs at FNB Stadium on January 5.
"I am still out for six more weeks. It's a grade two/three knee injury," Onyango said.
"But I am getting strong day by day because I do rehabilitation every day with the physio.
"I am hoping to be back before the six to eight weeks and get ready for the second round."
Downs are currently in Egypt for a CAF Champions League first round, first leg tie against Libyan side Al Ahly Benghazi in Cairo on Sunday.
Coach Pitso Mosimane, who has Kennedy Mweene and Reyaad Pieterse as his other options in goal, is also without injured trio Themba Zwane, Rivaldo Coetzee and Sibusiso Vilakazi.