The 63-year-old says a conversation he had with Gerard Pique over the midfielder's future should not have been made public
The midfielder has been linked with a move back to England this summer with Chelsea and Premier League champions Manchester City believed to be interested in securing the midfielder’s signature.
And Fabregas’ compatriot Pique appeared to confirm Fabregas would be leaving Barcelona in the coming months during a press conference with the Spain national squad this week.
The defender was caught on microphones telling Del Bosque "We have to defend Cesc a little. He told me it's done. He's off, €33m he said to me."
Pique shielded his mouth as he made the comments despite the press watching on and Del Bosque has now revealed his displeasure that the conversation between the pair was subsequently published.
"I think it was unfair that the conversation was leaked," Del Bosque told Radio Marca.
Fabregas came through the youth ranks at Camp Nou before joining Arsenal as a teenager, only to re-join Barca in 2011, but has failed to cement himself as a favourite among the Camp Nou faithful.
However, Del Bosque refused to expand on the player's next move and simply praised his performances for the national side in the build up to the World Cup.
"It's not for me to assess the club's decisions. He's a Spain player and I must say he's training brilliantly. He's really useful anywhere on the pitch,” Del Bosque said.
Spain will play their opening World Cup game against the Netherlands on June 13.