Sem Robberse
Sem Robberse is the best option available internally to replace Steven Matz and Matthew Liberatore as the fifth starter. Robberse has done all he needs to in the minors, and it's nearing time that he gets his chance in St. Louis with the big league club.
The Cardinals acquired the Dutchman when they traded Jordan Hicks to the Toronto Blue Jays at last year's deadline, and Robberse has not disappointed. The right-handed starter has pitched exclusively in Memphis this year, and he has a 3.29 ERA, .210 batting average against, and 1.04 WHIP in 54.2 innings (9 starts) this year.
Robberse started off the season as one of the organization's best minor league pitchers in April. He had an 8-inning, 9-strikeout outing on April 19th, and he followed that up by pitching 7 innings and only allowing 2 runs to score on April 26th.
May has been a bit of a struggle for Robberse, as he has two outings where he's given up 6 runs -- one with 4 home runs on May 8th. His 5.04 ERA in May is a bit concerning, but he has still been able to put together an admirable season.
Robberse last pitched on May 19th, so he is perfectly lined up to slot in as the team's fifth starter. Calls for Sem to be promoted have only grown in volume since Matz went down, and it's seeming more and more likely with each passing day that he will be chosen to pitch this next time around the rotation.