Rachubka, 31, joined the Reds last the weekend after a shoulder injury ruled out number-one 'keeper Ian Dunbavin for the rest of the season.
He met his new team-mates only hours before the FA Cup tie with Oxford and went on to make a string of impressive saves in the 3-3 draw.
Football League rules governing loans outside of the emergency loan window dictate deals can only be extended by seven days at a time.
This means Rachubka is eligible to face Burton on Sunday and can take part in the second-round replay at Oxford next Wednesday before further renegotiation.
Meanwhile, Dunbavin, 32, is set to undergo shoulder surgery on Thursday.
On his Twitter page this week, the keeper appeared to be remaining in good spirits about his injury.
He tweeted: "Thanks for all your messages of support. Once I've had my op on thurs it's going to be a long road back! Looking forward to eating Christmas dinner with one hand."