Full-back Scott Hamilton believes Leicester Tigers will be out of Europe after two just games if they lose at home to Ospreys on Sunday.
The 31-year-old Kiwi says a second defeat in the competition will be too much for his side to come back from, after Sunday's 23-9 reverse in Toulouse.
Tigers have been in this position before and still managed to qualify for the quarter-finals.
Famously, in 2007, they lost to Munster in their opening fixture before winning the next five on the bounce to go through.
Leicester have also had some memorable battles with Ospreys in recent seasons, and Hamilton says he is hoping the situation in Pool Two will help get the best out of his side.
"Being honest, we have to win our next five games in this competition," said Hamilton.
"If we lose on Sunday, we are out, unless we pick up lots of bonus points in the other four games.
"It's a tough ask but it's a must-win game on Sunday, and then we have to make sure that we beat Toulouse at home when they come to Welford Road.
"Losing to them was disappointing, but we had a long flight home and have a full week's training ahead of us to think about what to do better next time. Bring on the Ospreys."
Hamilton had a superb game in his preferred position of full-back at the Municipal Stadium.
But he was left feeling 'what might have been' after helping Tigers to a 9-6 lead 35 minutes into their Heineken Cup opener.
"We dominated up front in the first half and were on top in the scrums," said Hamilton.
"We were feeling okay at half-time and, even though they scored a try before we went in, the chat was good and the start of the second half was good, too.
"But it didn't go so well after then. Our discipline let us down and our scrums started going down. I don't know what happened there.
"They are a good side and are big lads. They played some good off-loads and kicked their penalties when they came. When you do that well, it's a pretty simple game.
"We didn't kick enough ball and we missed out on points at times.
"We just can't afford to miss out on that when the Ospreys come."
The Welsh region began their Heineken campaign last Friday night at home to Italian side Treviso.
Ospreys picked up a four-try bonus point, but only with their final score coming in second-half injury-time.
