If Shanahan were to come here, he'd be a 3rd line/4th line guy. Pesonen is a huge "if" to make it in the NHL right now. If we can get Cooke to play with Sid and move Dupuis down to the third line, Shanahan would be fine for 4th line duty, while seeing 1st/2nd PP unit time.
We do not have Malone anymore for the PP to stand in front of the net. Fedotenko isn't bad but I suspect that Staal will be on the PP unit with Sid and Satan. If Shanahan were to come on board, he could be on the PP unit with Malkin and Sykora, or interchange Staal and you have 1a and 1b PP unit.
Shanahan can still be a monster on the PP unit but he has nothing else to give. His legs are done. At least Roberts would play solid forechecking. Shanahan can't even do that anymore.