Rangers came from behind to claim a 4-2 win over the Steelmen, who finished the match with 10 men after Slattery was red carded for catching Cantwell in the face with a swinging arm.
Former Rangers defender Richard Foster, now head of coaching at Motherwell, hit out at the January arrival from Norwich as he accused the Ibrox star of play-acting in order to get Slattery his marching orders.
Speaking on BBC Sportscene, Foster said: "This one is an absolute embarrassment from Cantwell. His reaction is an absolute embarrassment.
"Nick Walsh has every right to believe that he's been elbowed really hard in the face. I don't think Tyson Fury would knock him down as quickly as that! He throws his hands to his face and then throws his whole body down back the way.
"Players need to take responsibility as if that is the other way around, everyone in the Rangers team complains. He does the referee and that takes my Motherwell bias out of this."
Motherwell manager Stuart Kettlewell criticised the role of VAR in the red card incident and also over Rangers third goal, which was scored by Cantwell, with suggestions that Fashion Sakala was offside in the build-up.
“I thought the way the last ten minutes played out were harsh on us,” said Kettlewell. “I mean by that the Sakala offside for the third goal which was a massive moment in the game. I’d also like clarity on Callum’s second yellow card. There’s contact but he’s trying to protect the ball and he’s not got awareness of the guy behind him and the contact’s made on Cantwell’s shoulder.”