Do you know how ******* old Strider is in The LOTR? 87.
Actually, now that I look into it, he would be only 10 when The Hobbit takes place. So even if Aragorn did show up, Mortensen definitely wouldn't be back. And he couldn't even play his own father, as he had died when Aragorn was 2.
So it looks like Mortensen is definitely out ... unless they wanted to really screw with the timeline.