Both players declined to speak with the media following Thursday's game.
According to Riggleman, Olsen felt tenderness in the arm during the All-Star break but did not think he would have to miss a start.
But during a workout on Wednesday evening, he did not feel good throwing the ball. When he tested the arm again on Thursday, he did not feel any better.
Stammen enters Friday's game coming off two of his best three starts as a pro. At Colorado on July 6, he absorbed a loss despite allowing just one run on five hits in seven innings.
He followed that performance with another road gem, throwing a complete game of two-run ball in a win at Houston on Saturday.
Olsen's first year with the Nationals continues to be rocky. He struggled to begin the season before going on the disabled list for more than a month with shoulder tendinitis. He made three starts since returning on June 29 and pitched well during that time, indicating that his injury troubles had passed.
The Nationals will have to play a game of wait-and-see and decide by the end of the weekend whether this latest malady requires another stint on the DL.
WSH: RHP Craig Stammen (2-4, 4.45 ERA)
Stammen seems to finally be "getting it" as a Major League pitcher. Following the hard-luck loss at Coors Field, the rookie scattered nine hits and walked one while allowing two runs to the Astros in a 13-2 blowout. He has gone deep into games recently because of his ability to throw his sinker for strikes early in the count.
CHC: RHP Carlos Zambrano (5-4, 3.53 ERA)
Zambrano picked up his first win since June 5 in his last start, on Sunday against the Cardinals. He gave up three runs on five hits and two walks over six innings while striking out seven. He also hit his third homer of the year, and the 19th of his career. He is 1-1 with a 1.86 ERA in three July starts.
After his 11th multi-hit game of the season on Thursday, infielder Alberto Gonzalez is now hitting .394 since being recalled from Triple-A Syracuse on May 30. Recently, Gonzalez, who is a shortstop at heart, has earned the bulk of his playing time at second base -- a position where the Nationals have had some instability this season. ... Going 6 2/3 innings and giving up three runs (two earned) on Thursday, John Lannan had his 12th quality start in 18 tries. During Lannan's career, the Nationals are just 15-20 when he has a quality start.
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Official game notes
WFED 1500, SBN 1390 (Español), ESPN 730 (Sp.)
Saturday: Nationals (Jordan Zimmermann, 3-4, 4.50) vs. Cubs (Ted Lilly, 9-6, 3.18), 7:05 p.m. ET
Sunday: Nationals (Garrett Mock, 0-2, 6.92) vs. Cubs (Randy Wells, 4-4, 2.72), 1:35 p.m. ET
Monday: Nationals (TBD) vs. Mets (TBD), 7:05 p.m. ET