If this plant were mine, I would sit it in a pot on a bed of damp sphag. I would pull the sphag, loosely, over the roots. You will probably need to spritz the sphag daily to keep it moist, not soppy. The remaining roots will "green up" with the moisture. This method is a bowl version of a mounted orchid where it is very easy to monitor the moisture level and the root growth.
As Mayres said, Superthrive on the sphag will help the root growth and encourage new roots. Yes to cutting the spike off as soon as it grows a little longer. Upping the temps is also needed since most phals don't like it under 65 degrees.
This is the hard part but I promise you it works. To keep your healthy leaves from dehydrating from the root loss, take the plant, turn it upside down and soak them in whatever fertilizer you use for 10-15 minutes. When finished, make sure the crown is dry and put it back on the moist sphag bed. I do this soak method once a week with whatever fertilizer solution I am currently using - in winter it is 1/4 strength and during the growing season 1/2 strength.
Here is a picture showing what was a totally rootless phal I have resurrected with this method.
During this process, the phal has grown two new leaves and only lost one leaf and they leaves were never limp.
Brooke