The Welsh region were beaten 29-12 at home by Racing 92, their second defeat in this year's tournament.
It leaves them seven points behind Northampton and four behind Racing, who have played only one game in Pool 3.
"It makes it very tough [to qualify] and they'll have to be a lot of turnarounds," said Pivac.
"We came up against a big, heavy pack and it's shown us the standard we've got to aspire to.
"It keeps us grounded, I guess, to see we have a long way to go to get to that standard, and we know that and we've just got to keep working hard.
"We'll see how it goes over the next couple of weeks and we'll see how we go into the Glasgow game looking to get a win."
Scarlets face Glasgow in their next two pool matches, before a home tie against Saints and a trip to Paris.
Pivac's team are among the early season pace setters in the Pro12, lying third behind Connacht and Munster.
But they showed little of their domestic form as Racing pounced on turnovers and defensive slips to claim a bonus point before half-time.
Even after the visitors' flanker Bernard Le Roux was sent-off for a stamp early in the second half, Scarlets struggled to impose themselves on the French side.
After facing the classy Parisians, Scarlets take on Italian strugglers Zebre in the Pro 12 on Sunday, 29 November.
"We've got to re-focus on the Pro12 and improve against Zebre in a week's time," added Pivac.
Scarlets coach Wayne Pivac admits his team face a "very tough" task to qualify for the knockout stages of the European Champions Cup.