Not a particularly appealing bet. Millwall might have squeezed past a dreadfully poor Aston Villa side, but they're average Championship at best and look short on firepower having lost the two forwards recently.
Luton have been like this for a few years now. They struggle to win the league (as they probably should) at least in part because the fans expect too much. They get on the players' backs early in games and the board/management are too trigger happy in their transfer dealings and team selection. But the fact remains they pay decent money, have plenty of FL standard players and when Luton are the underdogs, the fans can give them good backing.
I think Millwall would be perfectly happy with a replay.