The defender hurt his shoulder in conceding the penalty which Peterborough scored in the 1-1 draw at the Walkers Stadium.
Brown was substituted at half-time in that game and remains a doubt. He will be assessed closer to kick-off with Bulgaria international Aleksandar Tunchev on stand-by to make only his second league start since January.
On-loan Sunderland striker Martyn Waghorn is pushing for a recall. Dany N'Guessan would be most likely to make way after he was substituted at half-time against Posh in a tactical switch by boss Nigel Pearson.
On the game at Vicarage Road, Pearson said: "It will be tough. We have played pretty well so far this season and I expect us to go there and do the same again. Whether we win it or not is another thing, but we will go and try to win the game.
"We will treat it like every other game. We will do our preparation on the opposition but, ultimately, it is about what we do ourselves. It is about getting the right team and making sure we play to our maximum."