Saracens won - but missed their chance to secure a Heineken Cup quarter-final place one week early.
After Ben Spencer's try and three penalties from the boot of Owen Farrell had given Saracens a 14-6 half-time lead, McCall wanted his men to hammer home their advantage.
But Biarritz came back at them with Dimitri Yachvili scoring all their points, including a charge-down try, to set up a tight finish.
Farrell kept his composure to land the decisive penalty three minutes from time and Saracens' 20-16 victory keeps them in control of Pool Five.
Biarritz's losing bonus point means the group will not be decided until next week - but Saracens can book their passage into the last eight with a single point in Treviso.
"We are pleased to have won the game," McCall said. "These kind of games are about getting the job done. There is a little frustartion it wasn't more simple than it turned out.
"There were times in the first half and the start of the second where we were very dominant and at 14-6 we had an opportunity to pull away and break them.
"We couldn't do that but the last 20 minutes characterised everything we are about as a club with the the work ethic and discipline.
"We are not quite qualified yet. That will definitely focus the mind for next week. It is very important we are professional."
McCall was full of praise for his pack and three young England stars.
Farrell has attracted all the headlines of late but full-back Alex Goode enjoyed another outstanding game and Spencer caught the eye at scrum-half.
"The reason Ben didn't make the Saxons is because the RFU like them to go through their age groups," McCall said.
"Ben has never represented England at anything and this is an opportunity to play for the Under 20s in the Six Nations."
Goode was named in the Saxons party but will be part of England's Six Nations squad after being called in to provide injury cover for Toby Flood.
And McCall believes England could even consider lining up with Goode and Ben Foden in the same back division.
"In the last six weeks Alex Goode has been as good as he's been in the last two and a half years. He is back to his best," McCall said.
"Whether Alex and Ben Foden can be accomodated in the back three I don't know. Ben Foden has ability to play on the wing."
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