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Saracens top the group and lie two-points clear of second-placed Biarritz Olympique, who travel to Vicarage Road in January, while the Ospreys face an uphill battle to qualify.
McCall's side's victory brought an end to the Ospreys 17-match unbeaten European record at the Liberty Stadium, which stretched back six years.
"This was a massive result for us," said McCall.
"It's a good win against a good team who were unbeaten on their patch for a very long time.
"It was a very different game to the previous week. It was very scrappy and we had to roll up our sleeves and dig in. Twice we were down to 14 men but we kept on working and got the win.
"These back-to-back games are pivotal in the group and the team that wins them usually prevails. It's big step for us but we know we still have two massive games to come."
Saracens slumped to fourth in their Heineken Cup group last season and have only once reached the knockout stages - in 2008 when they once again beat the Welsh region.
And while McCall is pleased with Saracens form and position he is more than aware that the job is not yet complete.
"We were disappointed with how we went in Europe last year and we've made a real point of going for it this season," added McCall.
"We've now won 22 of our last 25 games in all competitions. I don't think there are many teams in Europe right now with that sort of record.
"It was a big step for us against the Ospreys but we know we still have two massive games to come."
