Williams has denied the Premier League outfit have landed Popov despite widespread reports and Litex Lovech's website, which claims the 22-year-old had agreed a deal subject to obtaining a work permit.
Popov is believed to be on trial at Rovers and is to undergo medical tests over the coming days, but the striker, who can also be deployed on the wing, has not joined Sam Allardyce's side as yet.
Williams told the Lancashire Telegraph: "He is one of a number of strikers currently under consideration. We have not made an offer to his club."
Allardyce has been looking to bolster his attacking options this summer and so far the Rovers boss as only secured Manchester United's Mame Biram Diouf on loan.