Darvish returned to Dallas and was examined by back specialist Drew Dossett on Wednesday. The Rangers were told Darvish is OK and can resume throwing Saturday.
The right-hander, who was second in AL Cy Young Award voting in 2013, won't start Opening Day, the team previously announced. He likely will start the season on the 15-day disabled list.
It will be Schepper's first major-league start. According to Elias Sports Bureau, the only other player in the divisional era (since 1969) to make his first career major-league start on Opening Day was Fernando Valenzuela in 1981.
The Rangers said Nick Martinez will be their No. 5 starter to begin the season. He has pitched in just five games above Class A.
The Rangers also reached a deal with veteran right-hander Scott Baker, according to multiple reports.
Darvish missed his scheduled start Friday because of neck stiffness. He said he slept on it wrong and assumed he'd be back in two or three days. But he hasn't been able to throw, and as a result, he visited Dorrett on Wednesday.
Darvish's back bothered him late last season and he received an anti-inflammatory injection to help a nerve issue in the lower back right after the season. Darvish said the symptoms went away in mid-January, aided by the fact he altered his offseason regimen to avoid lifting heavier weights.
After an impressive 2013 season, in which he had a 2.83 ERA in 209 2/3 innings and a league-leading 277 strikeouts, Darvish was named the club's Opening Day starter and arrived at spring training ready to go. He didn't have any issues until complaining about the neck stiffness last week.
The Rangers have been hit hard by injuries this spring. Catcher Geovany Soto had surgery on a torn lateral meniscus Wednesday and will be out 10 to 12 weeks.
Information from ESPNDallas.com's Richard Durrett was used in this report.