The Senegal international will join Premiership newcomers Crystal Palace on a year-long loan despite Leeds making an offer on Tuesday.
The Leeds boss is now hoping to beat Championship rivals Cardiff to the signing of Albion's Gregan.
Cardiff have been discussing a deal with The Baggies but cannot reach agreement over a fee.
The Leeds boss is now hoping to beat Championship rivals Cardiff to the signing of Albion's Gregan.
Cardiff have been discussing a deal with The Baggies but cannot reach agreement over a fee.
The West Brom skipper, 30, has been a top target for Leeds for sometime but Cardiff are planning to table a £350k bid. But Baggies boss Gary Megson has told The Bluebirds to put up or shut up.
He said: "Nobody is going to manufacture a transfer. If they want Sean they should put in a bid."
Gregan joined West Brom for £1.5m from Preston in 2002.