In August 1965, Minnesota picked him from its AAA Denver club to replace injured player Harmon Killebrew.

[1] As sports author Jim Thielman notes, it was common at the time for baseball's commissioner to have input on World Series rosters, and Commissioner Ford Frick suggested the Twins bring their roster to 25 men by dropping a player who had not participated for the entire season.

As a late-season replacement who played in the outfield, where the Twins were well-stocked with veterans, Kosco was kept off the World Series roster.

He was traded again to the California Angels (for Tommie Reynolds) and to the Boston Red Sox (for Chris Coletta) before completing his professional baseball career with the Cincinnati Reds, with whom he batted .280 in 118 at-bats in 1973 on a team that won the National League West title but lost to the New York Mets in the NLCS.

Kosco went 3-for-10 in that Series, including collecting the only two Reds hits in Jon Matlack's Game Two shutout.