The veteran striker, who joined the Hatters in November, was a surprise omission from Mick Harford's starting XI, but Watson revealed Gallen's daughter was taken ill before the game.
"Kevin Gallen's little girl isn't very well at all so our thoughts are with him and his family. He is a big miss," he told Luton Today.
"Kev has been playing out of his boots playing in a variety of positions. He knows when to take that extra touch."