Mngoma, who joined Usuthu from Thanda Royal Zulu on a three-year deal in June, hurt himself when punching a ball in his side's 1-1 draw with KwaZulu-Natal rivals Maritzburg United on Friday.
The 26-year-old admits the setback is hard to take after being handed his club debut ahead of Tapuwa Kapini, who conceded three goals against Mpumalanga Black Aces the weekend before.
"I'm not so sure because my whole arm is bent after I punched the ball and I felt the pain and at this moment, it is very painful," says Mngoma when asked about the knock.
"I'm not so sure how long will I be out for.
"It will be a big blow for me because I have worked so hard behind the scenes for the coach to recognise me to play and now I just got a chance. It has come at a difficult time.
"I'm not so sure if the bone got broken or it's a fracture, but I think it was fractured."
Mngoma, however, has vowed to come back strongly and regain the trust of coach Steve Barker.
"As of now I signed a three-year deal with the club, and I'm 26 now, so I will be at the club until I'm 29 if I do stay here.
"It helped me a lot to be patient and also the new coach Steve Barker knows me because I have worked with him in the past, and it's great that he is here.
"I promise you... I will work hard to bounce back from the injury setback."
Mngoma joins John Arwuah and Marc van Heerden on the treatment table at the Durban-based oufit, while striker Bornwell Mwape is suspended for Tuesday's trip to Ajax Cape Town after getting sent off against the Team of Choice.