Williams have fufilled their contractral obligation of having a dullard over-rated Scottish driver who does TV work as their 3rd driver by signing Paul Di Resta up for the job.
When asked about how he felt about the role he apparently stated that it wasn't his fault that he was 3rd driver but was down to tactical errors by the team and mechanical failures. He went on to say for the next 20 mins that it was obvious he was a top driver and deserved a top drive as he once beat Vettel in a junior series. By this time all the press had fallen asleep.
He has also been announced as a pundit on Sky which has been celebrated by C4 as it may attract them more viewers.
(I know I'm very harsh on Paul Di Resta by the way but don't rate him).
If Massa were to hang up his helmet during the season it would be planned with the start of the European races or the mid-season break. Williams would then choose a good replacement, thereby ruling out Di Resta.
I suppose it depends on how well Lance Stroll does. If he shows some speed and Massa does decide to pack it in mid-season we may well see someone like Wherlein being placed into a Mercedes powered team or one of their other junior drivers. I can't believe di Resta would be first choice for them or for any other team come to that.