Before he became the greatest relief pitcher in the history of the universe, Mariano Rivera was a middling starting pitching prospect who struggled in brief big league trials. He made 10 career starts for the Yankees, all in 1995, and pitched to a 5.94 ERA and 1.68 WHIP in 50 innings. Yuck.
Rivera was mostly terrible as a starter, but he did throw one absolute masterpiece. On Independence Day 1995, he chucked eight scoreless innings against the White Sox, striking out 11 and allowing just a pair of hits. Here's the box score and, with apologies to Hawk Harrelson, here's the video:
Most closers are failed starters, and Rivera really failed as a starter in a very brief opportunity. Things did work out just fine for him, I'd say.