The Robins are rooted to the bottom of the League One table, and lost 3-1 to fellow strugglers MK Dons at the National Hockey Stadium on Saturday.
Onuora, taking charge of the side after Andy King was sacked, said: "Two away trips in the space of a few days was always going to be difficult, but we should have taken something from the game.
"I will be making a few phone calls to see if I can bring a player or two in.
"We are in a difficult position at the moment, but we will work hard and I am confident we can get out of it."