Racing are now out of the hunt on 12 points - Farrell's final last-gasp penalty denying them even a sniff of a losing bonus point - and Munster, who play Edinburgh at Murrayfield in their Round 5 game, are the only remaining threat for group honours.
""It was a fantastic performance - in the last four years we have been building in this competition and a win against a very good French side in France continues that," said McCall.
"But we still have an important job to do next week against Edinburgh to secure the Pool winners spot.
"We're very proud of the team after bouncing back from the setbacks in the first half of the early injury, the tries and a yellow card.
"We were off our game in that first half and not working hard enough. But we knew at half-time that if we rectify things then we could win the match."
The fiery contest in Nantes was settled by the immaculate goal kicking of Owen Farrell as he punished Racing indiscipline with a tournament record 10 penalty goals - not to mention the conversion of full back Chris Wyles' try for a 32-points haul.