Obviously it's on him to win a spot but I feel for Robertson. Should just send him down at this point and let him get focused on the Marlies and hopefully forcing his way back up.
He hasn't been good enough, but he also hasn't been terrible which is encouraging. In good spots, getting chances, seems faster and able to follow NHL play a lot better than he did before. However he is clearly rusty (of course he has barely played the last few years) and looks like he took shooting lessons from Jason Blake sniping into the goalies crest every shot he gets.
He can use some time with the Marlies to refind his shot, his game, what makes him special. I think actually before he had the physical tools but lacked the mentality, now he has the mentality but rust has taken his tools. It's encouraging to me that he now has everything he needs and just needs to put it all in the toolbox.
Go down to the Marlies and score 15 points in 10 games, get your flow and confidence back, and come back with everything you have learned when someone gets injured. This is not the end of the line for him, and as long as he understands all of this and has the right attitude he will be fine.