Walker's injury, after he went down awkwardly in a tackle with Senior, was a cruel blow with Shaun Briscoe, Ben Galea, Scott Murrell, Ben Cockayne and Jake Webster already absent for the Robins.
But the Robins regained the lead with quick tries from Clint Newton and Ben Galea, the latter capitalising on a horrible mistake from Warrington full-back Richie Mathers.