Green joined QPR on a free transfer from West Ham United in the summer and was expected to be first choice at Loftus Road this season.
However, QPR subsequently snapped up Julio Cesar and the Brazilian's move from Inter Milan saw Green dropped to the substitutes' bench.
An injury to Cesar at Sunderland last month, though, has handed Green an unexpected chance and the England international has started the last two Premier League games.
The 32-year-old's performances have impressed Nelsen and the defender is hoping the keeper can maintain his form for the division's bottom club.
"Rob has been great for us," said Nelsen. "His confidence would have been down with Julio coming in and taking his spot.
"But you never know when your chance will come. You have to grasp your opportunity like he has done.
"He has been brilliant and was immense at Wigan, and I hope he carries on with that form."
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