There is a possibility that Shields, experiencing right forearm discomfort after earlier issues with his shoulder, will be held back in Arizona for extended Spring Training, which would put him on the disabled list to open the season.
"I think he's going to be realistic," Scioscia said. "He understands four, five innings [of spring work] might not be enough. If he feels good and is ready, so be it. But when a guy is approaching the start of the season and is not where he needs to be ... it's a possibility for Shieldsy [that he'll stay back in Tempe]. We'll see where we are in the middle of the week."
Shields on Saturday firmly maintained that he'll be ready to go "when the lights come on" at the Metrodome on March 31. But Scioscia, who knows how fiercely competitive the durable right-hander is, said he'll monitor his condition closely.
"One thing you guard against is [players] trying to do something on Opening Day they're not ready for," Scioscia said.
The forearm tightness surfaced after Shields threw two innings on Wednesday in a Triple-A camp game. Shoulder soreness had held him out of Cactus League play for two weeks, and in two appearances, an inning each, he has yielded four runs and four hits.
"I just have to let it calm down," Shields said on Saturday. "I'll be ready on the 31st."
With Chris Bootcheck doubtful for the opener -- the middle reliever is scheduled to throw from a mound on Monday for the first time since straining an oblique muscle on March 2 -- at least one relief spot already appeared available to a list of candidates including Rich Thompson, Darren O'Day, Jason Bulger and Alex Serrano. Shields starting the season on the DL would create another vacancy.
Lyle Spencer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.