Maybe you could find the x/y-position of the span with Javascript and position another div with an increased y-position? I don’t think there will be an out-of-the box css solution to this problem.
Here is a quick example:
<!DOCTYPE html>
<html lang="en">
<head>
<meta name="viewport" content="width=device-width" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<style>
p {font-size: 16px}
.underline {text-decoration: underline}
.below_underline {position: absolute; font-weight: bold; font-size: 12px; display: inline-block; text-align: center}
</style>
<script src="https://code.jquery.com/jquery-3.4.1.min.js"></script>
<script>
$(function(){
$('.underline').each(function(){
var position = $(this).position();
var newElement="<span class="below_underline">what if we remove this?</span>";
$(newElement).css('left', position.left).css('top', position.top + 20).css('width', $(this).width() ).appendTo(this);
});
});
</script>
</head>
<body>
<p>The quick <span class="underline">brown fox jumps over the</span> lazy dog.
</body>
</html>
It has some flaws, like if the text does not fit on one line etc. etc. but this should give you a good start.