Arbeloa has occasionally looked shaky at right-back in Spain's three Group C games in Gdansk, particularly against forwards like Ribery with pace and trickery, prompting calls for coach Vicente del Bosque to replace him with Juanfran.
A far more attack-minded player who is solid in defence, Juanfran is better at forging down the flank and firing over dangerous crosses than Arbeloa, which would have the added bonus of forcing Ribery to drop deeper to defend.
However, Del Bosque is unlikely to gamble on a player who only made his Spain debut at the end of May and he will almost certainly keep faith with Arbeloa, set to make his 39th appearance for his country in Donetsk.
At a news conference at Spain's training base in Gniewino, northern Poland on Thursday, Juanfran gave his full backing to Arbeloa but said he was ready to step up if needed.
"He is my team-mate and I will defend him to the death," the 27-year-old said.
"Right now the coach has placed his trust in Alvaro for that position and I have enormous respect for that," he added.
"He has my confidence and the confidence of the whole team. He played well in the group stage in my opinion and had a very good season at Real Madrid."
BURST BLISTER
Whoever is tasked with marking Ribery, the world and European champions will have to be on top of him from the start as they seek a place in the last four against Portugal or the Czech Republic, Juanfran said.
Ribery limped out of training on Wednesday with a burst blister but a team spokesman said he should be fit as France seek to defend their unbeaten record against the Spanish in competitive matches, winning five and drawing one.
"At any moment [Ribery] can create a scoring chance for himself or provide an assist," Juanfran said.
"He is one of France's most important players and we just hope he doesn't have a good day and that we know how to stop both him and his fellow attackers."
Midfielder Javi Martinez said the mood in the Spanish camp was "very positive" and confidence was high after they topped Group C ahead of Italy, Croatia and Ireland.
"We have had tough moments of course but we knew it would not be easy," he said.
"We came through a difficult group in first place and I think we have drawn strength from that."
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