hardly the most reliable of sources but it would appear to be a direct quote from a Watford FC spokesperson:
http://www.borehamwoodtimes.co.uk/sport/sportroundup/display.php?artid=1491972&FROMPAPER=watfordobse
A Hornets spokesperson said: "We have yet to receive any notification or requests from the Premier League regarding the transfer of Steve Kabba to Watford Football Club from Sheffield United.
"We can confirm that there was no written clause in any documentation relating to Steve Kabba that stipulated he could not play against Sheffield United.
"All contracts are vetted and approved by the Premier League prior to the player's registration."
The club refused to comment further on the matter "for now" leaving unanswered questions about the posting on its website in advance of the Sheffield United game: "Ex-Sheffield United striker Steve Kabba is ruled out of the game - it was a feature in his contract when he signed for the Orns in the January transfer window"
http://www.borehamwoodtimes.co.uk/sport/sportroundup/display.php?artid=1491972&FROMPAPER=watfordobse