Northampton boss Jim Mallinder has warned his side that anything other than victory over the Ospreys could end their hopes of European glory.
Defeat with a losing bonus-point at French side Castres means Saints are already treading a tightrope as they look to finish top of their group.
And director of rugby Mallinder insists their games at Franklin's Gardens are going to be massive in achieving that, starting with the Welsh on Sunday.
"We have to have a massive week. Come Sunday we need to win at home," he told BBC Radio Northampton.
"In the Heineken Cup you can't afford to lose your home games."
The Ospreys come into the game on zero points after losing 19-9 to Leinster at Liberty Stadium last Saturday, with Saints clearly in better shape than their weekend visitors to the Gardens.
"It's six games and that's what you've got to look at," added Mallinder.
"We've had our first game away to the French champions.
"We haven't won, but we came close and got the point. So we're still certainly well in the tournament.
"Clearly Ospreys have got a good side - the front five in particular.
"It's a massive challenge for us but if you want to be successful in the pool stages you have got to win your home games."