The 20 year old signed a 3 1/2 year deal with the Millers today after a £300k transfer.
The left-back has made 25 appearances for the Pilgrims this season.
Warne was pleased to have got Purrington in and looks forward to his input for the long term.
"He's perfect for us and he's a good fit and we're really pleased he's coming in. I've said this before, but I want to hand this team or squad over to the next manager in better form than I got it and I think he is one of them who will be a good player for us in the next six months, 12 months, two years." Warne said.