The last time Manchester United faced Southampton in a Wembley showpiece was 1976's FA Cup final, a sun-drenched May afternoon won by a Bobby Stokes goal that United fans of a certain age swear to this day was offside. That was Southampton's last (and indeed only) major trophy and it was won as a second division club. United, for their part, had been in that division the previous season, a club in the doldrums after their glories in the 1950s and 1960s.
United's current version of being in the doldrums is having not won the Premier League since 2013.