The Premier League has told BBC Sport it is investigating Steve Kabba's move from Sheffield United to Watford.
Kabba missed Watford's game against Sheffield United
Kabba, 26, made the switch to Vicarage Road for £500,000 in January.
According to reports, the Blades insisted upon his absence in April's home match against Watford - which the visitors lost - as part of the deal.
The Premier League would not have sanctioned the transfer if United had requested a clause preventing Kabba from playing against them.
In May a similar probe was launched into goalkeeper Tim Howard's transfer from Manchester United to Everton.
Howard joined Everton on a season-long loan deal from United in the summer of 2006, which became permanent in February.
Premier League rules on loan deals prevent players from facing their host club but, once the Howard deal became permanent, he was eligible to face compete against United on 28 April.
Howard missed that game but, in a statement, the Premier League said neither club were involved in any wrongdoing.
Sheffield United and Watford were relegated to the Championship last season but the Blades are still waiting to find out whether their appeal against the drop has been successful.
The Blades want to be reinstated after West Ham breached player ownership rules and a decision is expected on Tuesday.