$(".display-player-box").each(function() {                     // start looping
    $selected = 0;                                             // set a variable to default
    $boxid = $(this).attr("id");                               // set id of element being looped as variable
    $(".removefromsquad").each(function() {                    // loop through another row of elements
        if ($(this).attr("class").slice(16, 17) == $boxid) {   // looking for part of it's class matching previously saved variable
            $selected = 1;                                     // if yes, update variable
        };
    });
    if ($selected == 1) {                                      // if variable was updated...
        $(this).addClass("selected");                          // add class to .display-player-box
    } else {                                                   // if not...
        $(this).removeClass("selected");                       // make sure it's removed
    }
});

$(".display-player-box").each(function() {
  var selected = false;
  var selfId = $(this).attr('id');
  $('.removefromsquad').each(function() {
    if ($(this).hasClass(selfId)) {
      selected = true;
    }
  });

  if (selected) {
    $(this).addClass('selected');
  } else {
    $(this).removeClass('selected');
  }
});