The Swansea-based region crashed to a opening defeat to Leinster at the Liberty Stadium on Saturday.
And Fussell admits the Ospreys must triumph at Franklin’s Gardens to keep any hopes of reaching the knockout stages alive.
“It is do or die,” said Fussell.
“Going away from home, we might be the underdogs — who knows? — but we have to go up there to win.
“It is early days, but we’ve both lost and so it is fair to say it’s a do-or-die situation.
“We have to put our best performance out there and hopefully come away with a win.”
The Swansea-based region have failed to cross the whitewash in their last two encounters.
Fussell admits the try-drought is concerning but insists it is not a major issue and one that can be resolved.
“I just don’t think our attack has turned up,” said Fussell.
“We lost our way and we lost our shape — I can’t say why. We had a great build-up and were ready to go. But on the day little things didn’t go our way.
“It was frustrating, but there’s no one person to blame. It was a team issue rather than an individual one and we all have to put it right.
“Not scoring tries over our last two games isn’t really a concern, because in the two matches prior to that we scored 12. It’s something we can put right and we will put right for this weekend.”
But Fussell is under no illusions of the challenge the Ospreys face from George North and company at Franklin’s Gardens.
“It’s going to be a tough old day at the office because Northampton are no mugs: they’re a quality outfit in the Aviva Premiership and have been tipped to win this competition or at least make the last four,” added Fussell.
“We don’t come across English clubs very often, so in that sense it’s kind of unknown territory for us. They all fit the same mould in that they are big, physical, confrontational guys, and you just have to match that.
“Northampton also have quality backs, having made good signings, with George and Kahn (Fotuali’i).”
