[3] Kainer was promoted to the Major Leagues along with pitchers Brian Allard and Jerry Don Gleaton when rosters expanded in September 1980.

[4] He made his Major League debut on September 6, 1980, pitching an inning in relief of Sparky Lyle at Milwaukee County Stadium.

Kainer felt that manager Don Zimmer locked into a starting rotation of Jon Matlack, Ferguson Jenkins, Rick Honeycutt, Doc Medich and Danny Darwin without giving him an opportunity to earn a spot.

The Wichita Eagle-Beacon blamed his downfall from being "one of the Rangers' better-looking minor-league prospects" to being out of baseball on the inflated offensive environment of Lawrence–Dumont Stadium.

"[10] A Cincinnati Enquirer article later named Kainer as anecdotal evidence of how Lawrence–Dumont Stadium's dimensions could derail a promising young pitcher's career.