"The expectations are good on Diego Costa and Navas," Del Bosque told a news conference on Thursday ahead of Friday's penultimate warm-up match against Bolivia in Seville (2000 GMT).
"Tomorrow they will have tests and on Saturday we will make a decision," he added.
Atletico Madrid's Costa has been suffering from a series of muscle injuries in recent weeks and limped out of Saturday's Champions League final against Real Madrid in the ninth minute.
Navas, of Manchester City, missed the final weeks of the Premier League season with an ankle injury.
Del Bosque has until June 2 to name his final squad for the World Cup finals, when Spain, who are also the European champions, will seek to defend the title they won for the first time in South Africa four years ago.
Del Bosque called up Barcelona winger Gerard Deulofeu, who was on loan at Everton last season, to replace Navas, and he could make his Spain debut on Friday.
- Sports & Recreation
- Diego Costa
- Del Bosque
- Real Madrid