Lucas Giolito in spring 2014. (Photo by John McDonnell/The Washington Post)

The Nationals’ minor-league affiliates began rolling out their season-opening rosters Tuesday, and the most significant assignment was one that isn’t yet. Lucas Giolito, the Nationals’ top overall prospect, and Reynaldo Lopez, another highly-regarded hard-throwing right-hander, are not on the Class A Potomac roster, where both are expected to pitch this season.

No need for alarm. Neither pitcher is hurt, according to Nationals assistant general manager of player development Doug Harris. The Nationals are cautious with young pitchers, not just those who have undergone Tommy John surgery, as Giolito did in August 2012. Both prospects hit their prescribed innings limits last season.

Giolito, 19, was shut down after 98 innings at Class A Hagerstown last year, his first full professional season. The 2012 first-round pick posted a 2.20 ERA. Lopez, 21, tossed 83 1/3 innings last year between short-season Auburn and Hagerstown. He posted a 1.08 ERA, rising quickly up the top prospects lists because of his high-90s fastball.

Normally, pitchers begin a throwing program during the offseason, but the Nationals asked Giolito and Lopez to do that instead in Viera under the supervision of coaches. So instead of reporting to Potomac when the minor league season starts April 9, they will be a few weeks behind their teammates as they build up their arms for the season.

Both were spotted in Viera last week, playing catch with each other with no visible issues. They’ll remain there during extended spring training before joining a minor league affiliate.

“We wanted to monitor everything they were doing,” Harris said. “Particularly because of the volume they had last year. There are no issues with them. They’re two very talented young arms and we wanted to make sure of the safety of their arms.”

Among other assignments of note is newly acquired right-hander Joe Ross, who will begin the season at Class AA Harrisburg. Ross, who was among Baseball America’s top 100 prospects this offseason, was acquired from San Diego in the deal that sent Steven Souza Jr. to Tampa. Lanky and athletic, with a powerful sinker, Ross finished last season in Class AA, too.

Right-handed starters A.J. Cole and Taylor Jordan will begin the season in Class AAA Syracuse, as will young left-handed bullpen options Matt Grace and Felipe Rivero, either of whom would be candidates for call-ups this season. Top position playing prospects such as catcher Spencer Kieboom, middle infielder Wilmer Difo,  and big third baseman Drew Ward will start the season at Potomac.

Below you’ll find the complete rosters for each of the Nationals four affiliates:

CLASS AAA SYRACUSE

Pitchers

Bruce Billings

A.J. Cole

Manny Delcarmen

Matt Grace

Rich Hill

Taylor Hill

Taylor Jordan

Mitch Lively

Rafael Martin

Scott McGregor

Evan Meek

Felipe Rivero

Catchers

Dan Butler

Steve Lerud

Infielders

Emmanuel Burriss

Cutter Dykstra

Josh Johnson

Kila Ka’aihue

Jason Martinson

Ian Stewart

Outfielders

Tony Gwynn Jr.

Mark Minicozzi

Caleb Ramsey

Derrick Robinson

CLASS AA HARRISBURG

Pitchers

Tim Alderson

Robert Benincasa

Robert Bleier

Abel De Los Santos

Paul Demny

Paolo Espino

Bryan Harper

Gilberto Mendez

Joe Ross

Sam Runion

Derek Self

Matt Swynenberg

Austin Voth

Catchers

Cole Leonida

Pedro Severino

Infielders

Rick Hague

Stephen Perez

Shawn Pleffner

Tony Renda

Adrian Sanchez

Matt Skole

Outfielders

Delta Cleary

Kevin Keyes

Randolph Oduber

Drew Vettleson

CLASS A POTOMAC

Pitchers

Dakota Bacus

Cody Davis

Brian Dupra

Jake Johansen

Nick Lee

David Napoli

Nick Pivetta

Brian Rauh

Manny Rodriguez

Blake Schwartz

Matthew Spann

Wander Suero

Justin Thomas

Catchers

Spencer Kieboom

Craig Manuel

Infielders

Chris Bostick

Wilmer Difo

Khayyan Norfork

Drew Ward

James Yezzo

John Wooten

Outfielders

Isaac Ballou

Rafael Bautista

Estarlin Martinez

Brandon Miller

CLASS A HAGERSTOWN

Pitchers

Connor Bach

Andrew Cooper

Sam Johns

Tyler Mapes

Robert Orlan

David Ramos

Luis Reyes

Mario Sanchez

Ryan Ullman

Phillips Valdez

Drew Van Orden

Joseph Webb

Austen Williams

Catchers

Raudy Read

Matt Reistetter

Infielders

Osvaldo Abreu

Austin Davidson

Grant Debruin

Jose Marmolejos-Diaz

David Masters

Brennan Middleton

Outfielders

Dale (D.K.) Carey

Jeff Gardner

Narciso Mesa

Wilman Rodriguez