The 62-year-old coach was left bitterly disappointed following the world and European champions' slip-up against South Africa.
The world and European champions suffered a surprise 1-0 defeat at Soccer City in Johannesburg, the scene of their 2010 World Cup final triumph over the Netherlands. The result followed a narrow victory over Equatorial Guinea on Saturday, and Del Bosque concedes la Roja have been well below average.
"It could be [our worst recent performance]," Del Bosque told Spanish television station Cuatro. "I think we can even say the defeat was well deserved. We started well but little by little they started to open us up and we allowed them time to counterattack.
"We came out with good intentions and fought to the end to find an equalizer so I can't complain about the players' attitude. [South Africa] got better as the game went on though."
Bernard Parker scored the only goal in the 56th minute, dinking over Spain goalkeeper Victor Valdes, who was forced off with a serious-looking knee injury later in the match.
Del Bosque's men nearly mustered an equalizer, with Raul Albiol only denied by an outstanding Itumeleng Khune save late on, but South Africa held on for the win.
The loss was Spain's first since the Confederations Cup final, in which it was defeated 3-0 by host Brazil.