Russell will have had a year dragging the latest Williams shitbox around the track and putting the Kubica return well and truly back in its box.
Stick him in that second seat, if you can handle being Hamilton's team mate in only your second year in F1 then you have the chops to go on and possibly win the big prize. If not then merc can go back to the drawing board while Hamilton is still around to win things for them.
Hamilton started his career up against somebody many consider to be one of the best drivers to sit in a F1 car and was able to more than hold his own. Sometimes you just have to take a punt.