Del Bosque hopeful over Costa and Navas
Spain's coach Vicente del Bosque (L) attends a training session at the Sport City grounds in Las Rozas, near Madrid on May 28, 2014 - by Dani Pozo
"The prospects appear to be good. On Friday the pair of them will undergo tests. Diego will have a radiology test, as for Jesus he seems to be fine," Del Bosque said at a press conference on the eve of Friday's friendly with Bolivia in Sevilla.
"We will evaluate their conditions and take a decision Saturday morning," added Del Bosque, who is expected to reveal his final 23-man squad for Brazil on Saturday.
Atletico Madrid striker Costa has battled an ongoing hamstring problem that limited him to just nine minutes in last weekend's Champions League final defeat to city rivals Real, a week after he was forced off 15 minutes into his side's title decider with Barcelona.
The head doctor of the clinic where the Spain team sent Costa for a scan warned on Monday that he would require at least two weeks of rest before he could return to action.
Meanwhile, Manchester City winger Navas has been sidelined with an ankle injury since mid-April.
Owing to question marks over the fitness of key players Del Bosque delayed naming his full World Cup squad last Sunday to allow himself more time to monitor their conditions.
Instead he announced a list of 19 players for the World Cup holders' friendly against Bolivia, leaving out players from the two Spanish teams in last Saturday's Champions League final.
Del Bosque said he would use his full allotment of players in Sevilla, downplaying suggestions that certain individuals would be auditioning for a seat on the plane to Brazil.
"Nobody has anything to prove, the guys who are here have already shown they deserve to be here," said Del Bosque.
Despite a busy end to the season for several of his players, Del Bosque was also upbeat about the health of his charges.
"The season ends and we're asking them to give an extra effort that could last 50 days, and I hope it lasts that long. But honestly we've found them to be in rather good shape."
Spain face Chile, the Netherlands and Australia in World Cup Group B. Their first match pits them on June 13 against the Netherlands -- their opponents in their 2010 World Cup final triumph.