The Atletico Madrid striker, who made his only appearance for Spain to date in March, has been troubled by a hamstring injury that curtailed his involvement in last weekend's UEFA Champions League final.
Despite having earned just one cap, Hierro reckons that Costa could play a key role in Brazil and that Spain will miss him if he loses his race to be fit.
"The loss of a player like Luis Suarez, Diego Costa or Radamel Falcao would be decisive," he told AS. "They're very important for their national teams and I wish they were all there.
"It would be good for the competition. Strikers are artists whose goals decide matches."
Hierro, who won 89 caps for Spain, has urged coach Del Bosque to take a gamble on the 25-year-old forward if he is close to fitness and is confident Spain can become the first nation since Brazil in 1962 to win back-to-back tournaments.
"There were four players that last time, it wasn't known if they would go," he added, referring to Jesus Navas, Fernando Torres, Cesc Fabregas and Andres Iniesta.
"And they were the stars who put the finishing touches before that (winning) goal.
"I'm optimistic. I think a lot of this generation."
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