The Spain coach remains tight-lipped on who will be his No.1 goalkeeper for Euro 2016 qualifying but says that he is happy with the Manchester United star's development
The Real Madrid skipper endured a disappointing World Cup as the European champions suffered a shock exit at the group stage, despite entering the tournament as holders.
Casillas, unlike stars including Xavi and Xabi Alonso, elected not to retire from international duty but Del Bosque is still eager to test his potential successors against top-class opposition.
"De Gea did well, we are all happy with him," he told reporters after the game. "We needed to give him a run-out and we took advantage of this game to field him in a big game."
However, the veteran coach refused to comment on whether Casillas would return to the team for Monday's Euro 2016 qualifier against Macedonia, replying: "We'll see..." when asked who might take the gloves.
Reflecting on the game, which was won by a late Loic Remy strike, Del Bosque is encouraged by how his charges fared and insists that he is not worried by their struggles in front of goal.
"There were a few positives to take from the game," he declared. "We were not able to win but we gave a good impression. In the first half we looked more serious than in the second half.
"After the break our game was not as solid but we created more chances. We elaborated play, enjoyed possession of the ball and managed the game without any problems; it has been a good test for us.
"Diego Costa showed what he is capable of and now we have to connect with him and work on our understanding with him. We will improve."