The Aviva Premiership heavyweights were well below their usual standards as they struggled to deal with their Italian opponents, who led 17-13 at half-time.
By that stage, Saracens had already lost Kelly Brown to a suspected broken fibula and Steve Borthwick to a calf injury, and the wheels appeared to be coming off their European bandwagon.
McCall's men dug deep in the second half, however, and eventually ground out a result which will see them have a home tie when the knockout stages commence in April.
"I thought we were miles off where we normally are, and especially in the first half," McCall said. "The intensity wasn't there, and neither were the standards we normally expect.
"I was pretty disappointed at half-time. But then in the second half we dug in, and I have nothing but admiration for the way we got the job done."
Saracens had gone into the game chasing a bonus-point win which would have absolutely guaranteed their status as third seeds behind Munster and Leinster.
But in the end they managed just two tries, one apiece for Mouritz Botha and David Strettle, with the rest of their points all coming via the boot of Owen Farrell.
Treviso also managed two tries courtesy of Robert Barbieri and Tommaso Iannone as well as well as 10 points from place-kicker Kris Burton, and they were unlucky not to score more as they dominated for long spells.
Indeed, this was very much a backs-to the-wall effort from Saracens, characterised by one outstanding tackle from Peter Stringer to hold up 6ft 4in lock Antonio Pavanello when a try seemed certain.
"Peter has been a great signing for our club," remarked McCall of the diminutive scrum half.
"He's been here for three months and has showed all his experience.
"He was one of the better players today. His work-rate and his aggression on our line was brilliant."
It remains to be seen whether Stringer will still be with Saracens when they contest the quarter-final post-Six Nations.
McCall was unwilling to look that far ahead, and instead preferred to focus on what he called a "massive result" for the club, given that they avoided a potential trip to either Clermont Auvergne or Toulouse
As for Treviso it is another year without qualification, but they at least went down with their heads held high.
"We have displayed a spirit this year which we haven't always shown," said captain Pavanello.
"We have taken another step and have earned respect with our home performances. We will continue to go forward."
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