"Stephen has had permission to talk to Aston Villa and we have got Pascal Chimbonda talking here," said the Rovers boss.
"I think we have to get a replacement before we let Stephen go so if Pascal gets done here I'm sure Stephen will get sorted out."
Allardyce hopes the former Wigan and Sunderland full-back can rediscover the form which led Spurs to pay the Latics £4.5million for the player in the summer of 2006.
"It is the versatility I have chosen to go with rather than look for perhaps an out-and-out left-back," said the Rovers boss.
"He has huge experience of Premier League football and he is eager and ready to get away from Tottenham because he has not played enough games for them and he just wants to play football.
"When you meet someone who tells you they just want to play football it is a good thing from the start.
"When you see what he has achieved in his Wigan and Tottenham early days we want him to come and emulate that at Blackburn whichever position we pick him in.
"We had to scurry around for the last few days and we think we have found a very successful and experienced player to come and fill the gap Stephen has left."
Allardyce did not want Warnock to leave but admitted once Villa made their interest known it was almost impossible for him to hang on to the 27-year-old.
"Losing one of your best players is always a big disappointment but circumstances are such that it had to be," the manager added.
"We think we have replaced him with as good as we could possibly find.
"When fans see Pascal playing they will see what a good player we have on our hands and I will make sure he comes in and plays to the level he can play at."
Allardyce said he had to be sure he had a ready-made replacement before allowing Warnock to depart as he did not want other selling clubs to take advantage.
"If we were to allow Stephen to leave we needed to know who was coming in immediately alongside because if we didn't people could start upping their prices thinking we had lots of money to spend - which we haven't," he said.
Allardyce, however, has dismissed links with Middlesbrough left-back Andrew Taylor.