Neill spoke with the Hammers on Friday with regard to a move to Upton Park, and has already confirmed he intends to sign a deal with the London club.
Hughes was forced to do without skipper Neill for their 3-0 win at Manchester City, a victory that enabled Blackburn to climb back into the top half of the Barclays Premiership table for the first time in three months.
But Hughes, who lost Holland defender Andre Ooijer with a suspected broken ankle during the game, insists he does not plan on letting his skipper leave unless he finds a replacement.
He said: “We will have to wait and see.
“I cannot really tell you one way or the other what is happening on that.
“We allowed Lucas to go down and speak to West Ham yesterday (Friday), and he had a medical while he was down there.
“As I have always said I want to know we are not going to be weaker as a consequence of Lucas leaving, so exactly as we were.
“We are not going to let Lucas join unless we get a replacement, and nothing as yet has been agreed.”
The Australia international confirmed his plan to join the Hammers in an interview with SydneyÂ’s Sunday Telegraph.
Neill said: “People who have raised their eyebrows at me for making this decision are in for a surprise because West ham have a shopping list of players which almost blew me away.
“It was an extremely hard decision but I know it's the right one. West Ham moved heaven and earth to sign me.
“I supported them as a kid and I know people are going to give me stick for not choosing them.”
Neill was persuaded to join West Ham by chairman Eggert Magnusson.
He added: “It took me all of two minutes to be convinced I wasn't making a mistake when Mr Magnusson told me about his plans for the club.
"What he said almost knocked me off me feet.
"He told me I'm the first of many (new signings) earmarked for the club.
"I can be part of something big here. West Ham are very well supported. They just happen to be at the wrong end of the table at the moment but I hope to play my part in helping remedy that.
"They were two minutes away from winning the FA Cup final last season and have not suddenly become a bad team.”
Neill claims he has a get-out clause built into his deal should the Hammers be relegated and admits: “It's a calculated gamble. It won't be a glamorous three months but once we secure safety people won't question my motives any more.
“I believe I will have the last laugh. People will soon be talking about West Ham in a very different way.”
I doubt Mark Hughes will find a replacement for Neill..
They should focus on the youths like Arsenal and play like Arsenal..