The retired midfielder will step down as Blaugrana trainer at the end of the season, having won three back-to-back La Liga titles and two Champions League crowns.
Del Bosque feels that Guardiola is practically irreplaceable, and believes the coach has been a great ambassador for Spanish football.
"It will be impossible for anyone to achieve what Guardiola has achieved at Barcelona," the 61-year-old told reporters at a press conference.
"When he started coaching Barcelona I had left Real Madrid, but I have a great friendship with him through the Barcelona players.
"Pep is a person who has been a credit for Spanish football and has left a legacy. Guardiola is unique in history, he cannot be matched by anyone."
Del Bosque went on to admit that Guardiola should be a front-runner for the Spanish national coach role in the future.
"I imagine that when the RFEF have to appoint a new [Spain] coach, they will look for the best and Guardiola is among the best," he added.
"But we have a large group of Spanish coaches and there are many skilled people that could lead the team."
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