Mock won the job even though he was hit hard in his last two spring starts, including a game against the Astros' Minor League team. He will pitch against the Mets on Friday at Citi Field in his regular-season debut.
Right-hander Livan Hernandez, who is considered the fourth starter in the rotation, will not be activated until April 11, when he faces the Mets. Hernandez signed a Minor League deal with the Nationals this past February.
Based on Spring Training stats, Olsen had the edge, but the organization was split between him and Mock. There are some who believe that Olsen needs to build arm strength in the Minor Leagues.
One baseball source said Mock was picked because of his potential as a starter. The team believes he will one day become a No. 3 starter. Mock was told that he was on a short leash and the club would quickly call up Olsen if he didn't produce on the mound. There are some in the organization who feel Olsen is no more than a fifth starter.
"It was a very tough decision, particularly when Olsen threw the ball well [Friday against the Red Sox]," manager Jim Riggleman said. "The consensus among all of us -- coaches, myself and Mike Rizzo -- was that Olsen is really getting close to being where he was prior to [joining the Nationals]. He is quite not there yet in terms of being 100 percent.
"Even though Garrett Mock had a little bit of a rough spring, we are still excited about what he could do for us. You never want to be so flippant about it to say, 'You could flip a coin as to which one you took.' Really, it wavered day to day as to which pitcher we were going to take. Both of them have options. Whichever one we took, it could be an ever-changing thing -- not just with them, but with anybody else in the rotation, with the exception Jason Marquis and John Lannan."
Olsen did not take the news well. He told a reporter, "If you want a comment, call my agent."
The Nationals had a chance to release Olsen this past Wednesday or pay him a full base salary, which is $1 million. Washington could have saved $750,000 had it let Olsen go, but the organization feels he has a lot to offer.
As for Mock, he sounded like a person who knows he needs to get off to a good start in order to stay in the big leagues. Mock is coming off a season in which he was 3-10 with a 5.62 ERA.
"The expectations are high," Mock said. "I expect a lot out of myself. It goes a lot deeper. There are a lot of things I expect out of myself. I don't feel any pressure. I know [the Nationals] believe in my stuff. It's time to get started and it's time to win."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.