The Owls originally included Michael Turner (Norwich), Daniel Pudil (Watford), Alex Lopez (Celta Vigo), Aiden McGeady (Everton), Vincent Sasso (Braga) and Joe Bennett (Aston Villa) in their squad to face the Robins last week.
Turner, Pudil, Lopez and McGeady started, with defenders Sasso and Bennett named on the substitutes bench.
However, Football League rules state that a team may only include five loan players in a match-day squad.
After discovering their teamsheet error around an hour and a quarter before kick-off, Wednesday, occupying the final Championship play-off position with five matches remaining, left out Bennett and drafted in Jack Hunt.
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League One outfit Coventry City also made the identical mistake in their loss at table-topping Wigan Athletic on Saturday.
As far as the conduct of matches go, the captain and manager of each team must hand copies of a list of names of players taking part in a match, including substitutes, to the referee and a reprensentative of the opponents in the presence of the referee in his dressing room at least an hour before kick-off.
“The person who did the teamsheet made a mistake and we corrected it,” Carvalhal told The Star following their 4-1 thrashing at Ashton Gate.
But a Football League spokesperson has since told this newspaper: “The League has been in contact with the club and have asked for observations.”
Should Wednesday be found to have breached the rules on loan players, they could be hit with a fine.
Eight years ago, the Owls forked out £2,000 after failing to comply with the regulations on loan players.
Wednesday named six loanees in their squad in a 1-1 draw against Stoke City at Hillsborough. Of the loan contingent in their squad, two were unused substitutes and only three were on the pitch at any one time.
The Potters felt particularly aggrieved as Portsmouth loanee Franck Song’o scored the Owls’ equaliser. Despite that mistake, Brian Laws’ side escaped a points deduction.
In 2006, Leeds United avoided a points penalty for a similar offence.