De Gea's strong club form included 18 clean sheets during the 2017-18 Premier League season, earning him the Golden Glove for the first time.
He was unable to keep Switzerland at bay in Villarreal on Sunday, though, spilling a shot from Stephan Lichtsteiner that allowed Ricardo Rodriguez to earn a 1-1 draw.
Despite that error the 27-year-old will travel to the World Cup as Spain's undisputed number one, and Del Bosque remains a big fan.
"David is a really complete keeper," the World Cup and European Championship-winning coach told Omnisport.
"He plays well with his feet, which is important. He is improving and will be better.
"He's been playing for five or six years in Manchester, that's a huge learning experience, same with the national squad."
De Gea and his international team-mates are third favourites to lift a second world title in Russia, something Del Bosque believes is warranted after their unbeaten qualifying campaign.
"Let's see. Spain has done well recently, they did a really good job in the qualifying games with great results, almost a perfect qualification," added the former Real Madrid boss.
"There is not any recipe to play the World Cup. The most important thing is to keep doing what we already have done in the last two years.
"I think he [coach Julen Lopetegui] has done a really good job making a really good group. And apart from that, he has done a really good job developing his own style."