Matt Williams updates the latest on Stephen Strasburg

ST. LOUIS - The Nationals decided to rest Stephen Strasburg a day after the right-hander's start was ended prematurely by upper back stiffness.

"He's not gonna throw today," manager Matt Williams said before tonight's game. "We'll let the medication kick in a little bit and see how he is tomorrow. As of right now, we don't have an update because he just hasn't thrown a ball yet."

Williams pulled Strasburg after throwing only 60 pitches through four innings yesterday. The pain Strasburg experienced was similar to a discomfort which bothered him during an abbreviated start on May 3.

"It's kind of just like this ball," Strasburg said Sunday. "It's strange. It's not a cyst. It's just kind of this little knot that is just like there."

Williams indicated he isn't sure what's causing the irritation to flare up again, but the Nationals plan to err on the side of caution.

"I don't know," Williams said. "They've done every test. They've seen everything that they could possibly see. It's just discomfort for him. The problem with starting pitchers and pitchers in general, and you have some sort of discomfort that comes from your body, it can alter your mechanics, which can turn into serious injury."

Williams said missing a day of throwing shouldn't throw off Strasburg's routine considering he's taken the next day off after high pitch count games in the past.

"The issue will be does he feel a couple of days after (he throws) and does he still have that discomfort. If that's the case, then we have to make adjustments (to the rotation) and see where we can go with it. But if he feels good, then there's no reason he can't get to a (bullpen session) and see where he's at."

Right-hander Aaron Barrett's season ended earlier this month after he was placed on the 60-day disabled list with a right elbow strain. Barrett sought a third opinion from famed orthopedic surgeon Dr. James Andrews last week.

"We're gonna make an announcement on it as soon as (Barrett) decides what he wants to do," Williams said. "Of course, there's options all over the place, so he's discussing those. He is taking all the information that he's got from everybody and will make a decision on what he wants to do with it. But nothing official yet."

Williams said surgery, rehab and PRP therapy are all considerations for Barrett. Barrett was 3-3 with a 4.60 ERA in 40 appearances out of the bullpen in his second season in the majors. A bicep strain in his throwing arm also landed him on the 15-day DL on June 12.

The Nats can expand to their 40-man roster beginning Tuesday. Dan Uggla (back spasms) traveled with the team and is likely to be added to the roster when he's eligible to be reinstated from the DL tomorrow. Tyler Moore (left ankle sprain) should join the team later in the week when his DL stint expires on Sept. 3.

Tanner Roark, who is starting tonight for Double-A Harrisburg, is eligible to return to the Nats on Friday. Williams indicated that the team plans to add a few additional pitchers as well.

Meanwhile, Gio Gonzalez looks to get back on track tonight after three straight rough losses where he surrendered 16 runs (14 earned) over just 12 1/3 innings. Gonzalez held the Cardinals scoreless over six innings in the Nationals' 2-1 win in D.C. on April 21.

Led by Matt Carpenter (20 homers, 69 RBIs), tonight's Cardinals lineup has four left-handed swingers facing Gonzalez. Gonzalez brings a 2-1 record with an 0.93 ERA in four career starts against St. Louis into tonight's game.

"With Gio, it's always about command," Williams said. "And if he can command, then he's really good. We know all the info that's out there ... spin rate on the curveball, how good it is. If he command it, then he has success. If he can command fastball on both sides of the plate, he has success.

"When he doesn't and ends up getting behind guys or walking guys, then that's when he has trouble. So hopefully tonight, the former is the case as opposed to the latter and he commands (his) fastball and uses his curveball tonight. It'll be especially important against the lefties in their lineup, too."