The Warriors must complete their Italian Job this Sunday and hope Perpignan swat the Wasps in France in order to reach the Heineken Cup quarter-finals.
After their victories over both Pool rivals, all eyes will be on the powerful Welsh region making meatballs of the Italians who are without a win in the competition so far.
But seasoned campaigner Howells knows they cannot take anything for granted.
"We have had some big games against Wasps and Perpignan during the last month but if we tail off in our performances then we will not qualify, it's as simple as that," he said.
"It won't matter what happens in Perpignan, we can't pay any attention to that. All I'm concerned with is Calvisano. People are talking about bonus points but we have got to win the game first."
The Warriors will be well aware of the threat the Italians pose having found themselves 22-20 down at half-time, after leading by 20, in their last meeting at Bridgend.
"I watched them at Wasps last week and for half-an-hour they were 6-0 ahead. So you should never underestimate them, they have some quality players who can cause us problems," said Howells.
"In a competition like the Heineken Cup you can't stop performing. The players know that and it's the message I will be drumming into them. What amazed me was that we managed to get a result against Perpignan last week on the back of a bad set-piece.
"The fact that we won is testimony to the character of the team and they will need that again, but we cannot afford to have another game like that."
Richard Parks and Ceri Sweeney are both recovering from knocks but should both be fit to play as Howells enjoys the luxury of a near full-strenth side.
That means lock Deiniol Jones gets the nod again alongside Wales star Robert Sidoli with the Warriors coach renewing his calls for Steve Hansen to consider the making them partners for the Six Nations.
Howells said: "People at national level should be looking at Deiniol. The sign of a good player is performing consistently well and I can't think of a game that he hasn't done so for us."
Warriors: G Wyatt; D James, S Parker, G Thomas, A Havili; C Sweeney, G Cooper; G Jenkins, M Davies, C Horsman, D Jones, R Sidoli, M Molitika, R Parks, C Harris. Replacements: M Rees, C Loader, B Cockbain, N Kelly, S Martens, N Jenkins, J Bryant.