In 2013, MLS introduced New York City FC as its 20th team, and Orlando City Soccer Club as its 21st team, both of which would begin playing in 2015. In 2013, the league implemented its "Core Players" initiative, allowing teams to retain key players using retention funds instead of losing the players to foreign leagues.  Among the first high-profile players re-signed in 2013 using retention funds were U. S. national team regulars Graham Zusi and Matt Besler. Beginning in summer of 2013 and continuing in the run up to the 2014 World Cup, MLS began signing U. S. stars based abroad, including Clint Dempsey, Jermaine Jones, and Michael Bradley from Europe; and DaMarcus Beasley from Mexico's Liga MX.  By the 2014 season, fifteen of the nineteen MLS head coaches had previously played in MLS.  By 2013, the league's popularity had increased to the point where MLS was as popular as Major League Baseball among 12- to 17-year-olds, as reported by the 2013 Luker on Trends ESPN poll, having jumped in popularity since the 2010 World Cup.