Nelsen has been appointed head coach of Toronto FC and the Major League Soccer club expect him to start work on February 1, ensuring Tuesday's Barclays Premier League clash with Manchester City will be his final game for Rangers.
When the 35-year-old announced at the start of the month that he would be leaving, Redknapp expected to keep a player he values highly until the latter stages of their relegation battle.
While he is resigned to losing him far sooner, he wants the New Zealand international to be available for the crucial showdown with Norwich at Loftus Road on February 2.
"I need to speak to Ryan, I need him here for the Norwich game," said Redknapp. "I thought we were talking March 1 when I spoke to Ryan last. Suddenly this has all changed. I'm hoping we can put it back a few days.
"He's been away all week in Canada doing some work for his new club I need him next week - we're desperately short and I need Ryan to play.
"He's got a job to go to and wants to crack on with that. He felt it's an opportunity that may never come along for him again.
"I can understand where he's coming from because it's his future, but I wish he was staying because there haven't been too many better players than him here this year."
Redknapp will play a weakened team against npower League One club MK Dons in the FA Cup on Saturday.
"You want to keep winning, that is the key. We've had a five-game unbeaten run and want to continue that," he said. "But we have Manchester City on Tuesday and Norwich the following Saturday, so the squad will get used, and that's no disrespect to MK Dons."
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