The match will have a significant bearing for both sides with Ospreys chasing a play-off berth and the Dragons looking to stop a poor run of form which has seen them pick up just seven points in the RaboDirect PRO12 in 2014.
But despite the lack of form, 29-year-old Coombs is confident that his side can cause an upset and claim their first win in five league matches."There's no doubt that we are the underdogs," Coombs told the South Wales Argus.
"That's disappointing because at the start of the season we had snapped out of that. Now we've fallen back and we have that tag again.
"It's tough at the moment. We're on a very bad run and need to snap out of it because we feel that we have improved as a side this season.
"We've got three games left and have to have a crack at all three. It's down to the players to do that, there's nothing that the coaches can do because it is those of us on the field that are responsible for this form.
"Our mistakes are killing us and from 1 to 23 we have to be a hell of a lot better on Sunday, but I believe that we can go out there and cause an upset."
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Guinness PRO12 Dragons News
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