The RaboDirect PRO 12 outfit are level on points with Sarries in Pool 1 after a 15-9 victory at Thomond Park.
Ronan O'Gara kicked all of Munster's points and watched as his opposite number Owen Farrell missed four penalty chances, although the England star did secure a vital late bonus point.
Munster head coach Penney is now looking forward to Sunday's encounter between the sides at Vicarage Road, with both outfits on ten points, two ahead of Racing Metro.
And he was also full of praise for the hosts' defensive display.
"They are all 'must win' games, we are in a tough pool," he said. "But Sunday's game is a must-win.
"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.
"Something we have been really focusing on is our discipline and our work with referees. We got to a point where we conceded four penalties last weekend and I think we were up at 18 or something this week."
Elsewhere, Connacht head coach Eric Elwood was left to reflect on claiming the sizeable scalp of Biarritz, after Dan Parks kicked 17 points in a 22-14 win at the Sportsground.
The result leaves Connacht with a shout of qualification from Pool 3 as Tongan winger Fetu'u Vainikolo scored a first-half try.
"It's a huge scalp for us," Elwood told the Belfast Telegraph. "Biarritz had a full squad out there and we had a lot of guys missing.
"A lot of credit must go to our young guys, particularly the forward pack.
"The platform from our forward pack was terrific. To get scrum penalties against Biarritz was magnificent and Dan Parks kept the scoreboard ticking over, so all in all it was great.
"Thankfully it finished in our favour and they got nothing out of the game. That is a good feeling, it would have been awful if they had got a bonus point at the end."
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