They are likely to play for the reserve team on Wednesday, and could come back into contention to play Crewe on Saturday.
"Williams and Mulligan were not considered for selection against Bristol City, as a result of both players breaking the clubís disciplinary code relating to the consumption of alcohol," said manager Nigel Adkins.
"The club have spoken with both the players and dealt with this matter internally. Both players will be available for selection for Wednesday's reserve team game.
"There will be no further comment on this matter."Nigel Adkins