I looked but couldn't find it. Seems to reference a baseball player who is good in spring training but not during the season
Here’s some history as a guide: Before becoming a starter, Blue Jays first baseman Willie Upshaw was a spring hero. Entering the 1982 season, he had a .186 average and six home runs in three regular seasons, yet he had batted .433 and .333 in back-to-back springs with 13 exhibition homers, first earning the moniker Mr. March.