A magnificent defensive effort, led by outstanding flanker Jacques Burger, laid the foundation for a 16-13 victory at Ospreys which puts Saracens' European destiny in their own hands when hostilities resume in January.
The Ospreys dominated territory and possession but were forced into countless errors by Saracens' suffocating performance without the ball.
The Aviva Premiership champions played 20 minutes of the second half with 14 men after Schalk Brits and Kelly Brown were sin-binned but only conceded seven points during that period to ensure they took the spoils and ended the Ospreys' unbeaten European home record which stretched back 17 games to December 2005.
Ernst Joubert got their only try as he capitalised when Charlie Hodgson charged down Dan Biggar's attempted clearance, with centre Owen Farrell adding the conversion and three penalties.
The win keeps Sarries top of Pool Five, two points ahead of French side Biarritz, and McCall was delighted with the result.
"That was massive for us," he said. "We spoke about needing to back up a good performance and a good win last weekend against a good team who were unbeaten on their home park for a very long time and we have done that.
"It was a different game to last weekend, it was crappy and we had to roll our sleeves up and dig in, I have nothing but respect for our group. We were down to 14 men for 20 minutes in that second half and they still worked and worked, they are a special group."
Biggar kicked two penalties for the Ospreys and converted a second-half try from lock Ian Gough, but they could not make good on their dominance and they are realistically out of contention for a place in the knockout stages.
And head coach Sean Holley rued the mistakes that had cost his side.
"A couple of errors cost us," he said. "To concede a try from a charge down was very frustrating.
"We dominated territory and possession and some errors at crucial moments cost us. But full credit to Saracens, they have an outstanding defence and we couldn't break it down.
"I can't fault the effort and attitude of the players, they went for it and didn't leave anything on the field. We couldn't ask much more than to tidy up some bits of accuracy."
But both coaches were scathing in their assessment of the scrum and the way it was handled by French referee Jerome Garces.
The set-piece was a mess throughout the contest with Garces awarding a steady stream of penalties and free-kicks against both sides, much to the bemusement of the six international forwards in the respective front rows.
The Ospreys had Wales prop Paul James sin-binned just before the interval after he was adjudged to have failed to bind, while Brits' yellow card was harshly handed out for an early engagement.
And Holley said: "I don't where we are going with the scrum, at one point there were 11 penalties from 12 scrums.
"We had three penalties in a row five metres out, and then one went against us. Nobody knows what is going on there and it's frustrating for players and fans and I don't know what else to say. Are we better off not having scrums?
"Referees have a difficult job to do and they are instructed to look at certain things. I asked at half-time if it was just a Paul James issue as it seemed harsh for one guy from 16 to be singled out.
"(Ospreys forwards coach) Jonathan Humphreys was pointing out the illegality and angle of (Saracens tighthead) Matt Stevens but Paul James gets sin-binned, I just don't know to be honest with you."
McCall added: "Schalk's sin-binning was the first time I have seen someone sin-binned for early engagement. The scrums were carnage tonight.
"In the old days you used to practice scrum moves but coaches have stopped doing that as there is a free-kick or penalty at every scrum so there is no need for those moves."
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