That's not entirely accurate. I was in the crowd at Backlash 2003 and it was a mixed reaction for both Lesnar and Cena. It wasn't really a hot match either. But Cena had started to get face reactions prior to that match. He was pretty over during the tournament leading up to it. The match wasn't too hot because everyone knew Cena wasn't in Lesnar's class yet.
Cena didn't start getting boos as a face until the feuds with Jericho and Angle, especially Angle because everyone was pissed that Cena was outwrestling an Olympic gold medalist