Pirro had stood in for Spies during much of his long absence with shoulder injuries in the first half of the 2013 campaign.
Spies returned to action at Indianapolis last weekend, only to sustain a dislocated left shoulder in a practice crash.
The American will be assessed again in Dallas on Tuesday to decide if he needs further surgery, and he is confident he will only be sidelined for a brief period.
"I'll try to come back as soon as possible, but I also want to come back at 100 percent," Spies said.
"There's no reason for me to rush back just to fill the grid, but definitely this injury is nothing like the right shoulder.
"The pain is very bad right now, but the healing process won't be anything near the other one - I know that for sure.
"This injury won't even affect me in a few months, but I need to do the right things the right way."
- Sports & Recreation
- shoulder injuries