Seixas rode a remarkable race as he settled into the role of podium contender, battling Tadej Pogacar, Jonas Vingegaard and Remco Evenepoel on high mountains and taking a fourth place overall.
However, Lefevere warned that other steps in his development must not be skipped and that there is no rush in helping him reach his potential.
Paul Seixas ends the Tour de France in fourth place
Lefevere would not have brought Seixas to Tour
"I'm very impressed. But personally, I might not have let him ride the Tour at only 19," Lefevere
told Cyclism'Actu."We saw that he started to falter towards the end, which is perfectly normal. He'll be a great rider, but we mustn't rush things. We mustn't skip any steps."
Asked if the Tour was a success, Lefevere noted his difficulties in the final stages but ultimately concluded that the Frenchman enjoyed a strong debut.
He said: "For me, yes. Finishing fourth in the Tour de France at 19 is exceptional. He suffered in the final days, but he had a very good Tour."
He wondered if Isaac del Toro's Tour would have finished different if not for teammate Tadej Pogacar's assistance. After a white jersey and third place finish, del Toro has now finished on the podium of two grand tours.
Lefevere on del Toro and Evenepoel
Lefevere said: "I think so. But we mustn't forget that Tadej helped his teammate enormously. Without him, Del Toro probably would never have made it onto the podium and he likely wouldn't have won his first stage."
His former rider Remco Evenepoel put in one of his career-best performances, effectively ending any debate on whether he can be deemed a general classification contender at a grand tour. Lefevere insisted that a Tour de France win is not out of his reach, especially given his relative youth compared to Pogacar.
He added: "I must admit that I wasn't entirely convinced he was a rider capable of competing for the overall classification over three weeks. But he proved us wrong. Tadej is human, despite everything. Remco is two years younger; he needs to keep progressing and persevering."