Following their exit in the CAF Champions League group stages after a goalless draw against Horoya FC on Tuesday, Downs now need to overturn the 1-0 defeat they suffered in the first leg against City.
"Of course there is a chance [of winning the trophy] but we are looking at it as half-time. They are leading at the moment and we hope for the best in the return leg," Onyango said.
"This is the one cup we have not won as a team. Personally, I have never won it as well so I am looking forward to the match on Sunday."
The Ugandan is determined to keep a clean sheet against Benni McCarthy's visiting side in Tshwane.
"At the moment we are looking for a goal. We are looking at a way to score and not to concede, because if they score one then it becomes more harder for us with the away goal."
With their Champions League disappointment still fresh in their minds, Onyango insists winning the MTN8 would not be viewed as a consolation.
"We are looking at winning the competition as the trophy on its own because we never won it.
"We need to keep the team going. If we win this competition it will give us a boost ahead of the season."