Left-hander Robbie Ross will be recalled from Triple-A Round Rock to replace Mikolas. The Rangers have to make a roster move to make room for Ross.
Mikolas (2-5, 6.44) has been dealing with shoulder issues recently but has pitched through it. He tossed eight innings shutout innings on Monday night at Seattle, earning his second win of the season. He struck out five while throwing 92 pitches. Mikolas was able to pitch his normal between starts bullpen session but reported some shoulder fatigue afterward.
Rangers manager Ron Washington said there are no plans to shut Mikolas down for the season.
Ross threw 42 pitches over 2 1/3 innings in a relief appearance on Thursday for Round Rock. He gave up two hits and a walk with four strikeouts. Washington said Ross should be fine to start, despite the number of pitches thrown.
This will be Ross' third stint with the big club in 2014. He was demoted after an Aug.14 game where he gave up six earned runs in 4 1/3 innings in a 6-3 loss to Tampa Bay.
On the season, Ross is 2-5 with a 6.06 ERA. Ross was mainly used as a reliever but was able to get several starts, where he prefers to stay. He's 1-5 with a 5.40 ERA in 53 1/3 innings as a starter this season.
Mikolas becomes the second Rangers pitcher this month to get scratched from a start. Staff ace Yu Darvish has missed the last three starts with inflammation in his right elbow. He currently sits on the disabled list.