After the highs of the Six Nations, Halfpenny was brought down to earth with a bump at Firhill where Glasgow's 31-3 victory all but ended their RaboDirect PRO12 play-off hopes.
That leaves the Heineken Cup as their only realistic avenue for silverware this season but that equation is far from simple with a visit to the Aviva Stadium to face champions Leinster.
And while they have already been written off in some quarters, Halfpenny admits they cannot afford a repeat of their Glasgow performance if they are to have any chance.
"Glasgow was a tough place to go," said Halfpenny. "We just weren't at the races really.
"It was probably our worst game of the season. We have got to put that behind us and make sure we improve on the areas we were lacking in for this week's match.
"It wasn't a good day for us. We were extremely disappointed, but we have got to use that now to really take us into this weekend.
"People are writing us off, saying we have the odds stacked against us and we haven't got a chance.
"But we have got to take that as motivation and just go out there with a belief that we can get a result.
"We've got to go out and do the same as we did with Wales, which is do the basics well and keep things simple."
While Wales' star Jamie Roberts picked up a knee injury against the Warriors which will rule him out of the remainder of the season, Blues coaches Justin Burnell and Gareth Baber are boosted by the return of key men Gethin Jenkins, Xavier Rush and Martyn Williams.
Dafydd Hewitt replaces Roberts while fly-half Dan Parks, flanker Michael Paterson and hooker Marc Breeze also start and Halfpenny admits the cavalry couldn't have returned at a better time.
"They have come back in and brought a lot to the party," said Halfpenny.
"They have brought a lot of experience, a lot of communication on the pitch and just a lot of knowledge and things that we need to do well to get a result.
"They have all been there and done it and it's great to have them come back into the team and really strengthen the squad."
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