Pool Four leaders Munster, who have already secured a quarter-final place for the ninth successive season, boast an imperious undefeated tournament record at Thomond Park, scene of Saturday's showdown.
Leicester, though, must end that sequence to guarantee themselves a last-eight spot, although a losing bonus point could still see them progress as one of two best runners-up.
The Tigers lost 21-19 at home to Munster three months ago, when fly-half Ronan O'Gara landed a late 50-metre penalty in terrible conditions that left Leicester knowing any further losses would mean likely elimination.
Scrum-half Ellis, who could strengthen his claims for the England number nine shirt against Scotland on February 3 by dominating the battle with Munster's Peter Stringer on Saturday, accepts judgement day is looming.
Ellis said: "It is the biggest game of the season - how often do we say that? - but it is true of this match because we have to go to Munster, win and win well.
"The way these pools have gone in the last two to three years, it has been really tight and come down to the last round of games.
"To go there and win would be tremendous, and it would, hopefully, be a real springboard for the rest of the season.
"They are very passionate about their rugby over there, but we are determined and we will be switched on to giving them a good fight.
"Munster do the simple things really well. They are very direct, control the game well and kick to the corners to play a territory game."
While Leicester have injury concerns surrounding fly-halves Andy Goode, Paul Burke and Ian Humphreys - former Munster player Burke could start on Saturday - the Irish province report no major fitness problems as they target a 14th successive Heineken Cup win.
Munster are unbeaten in European competition since Sale Sharks toppled them at Edgeley Park in late 2004, although they almost came unstuck against Bourgoin four days ago before prevailing 30-27 in Geneva.
Munster skipper Paul O'Connell said: "There are things we have to refine and tidy up after that game.
"We looked at attacking space against Bourgoin - and the boys did that - so we just have to keep on doing what we are doing and tighten it up a bit. There was a lot of good stuff in the Bourgoin match, but at the same time, it is a long way off the type of rugby you need to beat Leicester.
"Mistakes can happen when you have got a lot of hands involved, and we will have to tidy up a lot. I don't think a high-risk game will suit when we play Leicester.
"Leicester are coming to our ground and they have to win. They have clever, experienced players who find that it helps their game a lot to be in an all-or-nothing situation, rather than doing the maths throughout the match.
"It puts you in a unique situation where you have to go all out and play, so it is going to be very different to normal games.
"It is not a question of of individual danger players when it comes to a team like Leicester, as they are a very complete side. They are a well-coached, tidy, efficient, clinical team, and I think those teams can be the most dangerous to play."