Factory test rider Pirro was originally listed as the stand-in for Spies in the Spanish Grand Prix, as the American skips the race to recover from lingering injuries.
But Jerez had previously been announced as one of the venues at which Pirro would ride an additional works Ducati alongside Andrea Dovizioso and Nicky Hayden, and Ducati said earlier this week that this would go ahead, with the Italian on a development machine.
Pramac will therefore run just one bike in Spain.
Spies has been struggling for fitness since a crash at last year's Malaysian Grand Prix, when he was part of the Yamaha works team.
He was forced to sit out the remainder of the 2012 campaign, and then exacerbated his upper body injuries in the last round at Austin.
"I am doing my absolute best to get back to 100 per cent, where I need to be, for the team and Ducati," said Spies.
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- Nicky Hayden