Youngsters Vinicius Junior, Marcos Llorente, Federico Valverde and Javier Sanchez de Felipe were all given starts by former Castilla boss Santiago Solari at Santiago Bernabeu, but Carvajal was unimpressed by their performances.
Speaking to Movistar+, the 74th-minute substitute said: "Three weeks ago there was talk that the homegrown players could play.
"But it turns out that they might not worthy of the first team."
Midfielder Isco was jeered by home supporters after he missed a clear shot at goal, leading the 26-year-old to turn to the crowd and reportedly say, "What do you want me to do?"
Speaking about the incident, Carvajal added: "I have not seen if he was whistled by the fans or not, but the only thing I can say about Isco is that he has been here for many years and he is a brave player.
"When things are not going well, he asks for the ball and tries to change the game rather than disappear."
Carvajal claimed the squad would move on quickly from the defeat and defended his manager's team selection.
"This was an inconsequential match for those who want to see it like that. It's an official match, it was not a good match. The first part we had enough opportunities but we had defensive errors and CSKA Moscow condemned us," the right-back continued.
"We're frustrated by the defeat at home but we finished first in the group and that was the objective.
"These are evaluations that the coach has to do, we are all fit to be here but it is difficult for Solari."