Munster head coach Rob Penney was already looking to the return clash after beating Saracens 15-9 at Thomond Park.
The Irish giants are now level on points with the Londoners in Heineken Cup Pool One after Ronan O'Gara's five penalties secured a hard-fought victory.
Sarries should have got closer to their hosts, but England fly-half Owen Farrell endured an off night with the boot, although his late penalty did secure Mark McCall's men a bonus point.
Penney said afterwards: "They are all 'must-win' games to be fair. We are in a tough pool. But Sunday's game (at Saracens) is a must-win.
"It is something we have been really focusing on, is our discipline and our work with the referees," he added.
"We got to a point where we conceded four penalties last weekend and I think we were up at 18 or something this week.
"Both sides defended so well and the pressure of the occasion probably didn't let the skills unfold as well as both sides would have liked."
McCall said of his visitors: "We got off to a poor start and Munster were obviously very motivated. We lost a couple of set-pieces during that time. It gave them a lot of momentum.
"It was a frustrating kind of game because when we did have possession they were quite happy to give penalties away.
"The penalty count was huge against them. It was some consolation that we got a bonus point at the end."