The line factors in Del Po's injury so it makes sense. I think the odds makers are tipping their hat here towards Ferrer, as it is begging for money on Del Po.
Del Po doesn't move well at the best of times and Ferrer is one of the best players on tour at making you move. It's why he performs so well against Del Po and other larger players such as Berdych. Only players with a combination of elite stroke play and movement (ie the top 4) have his number.
With a book like
pinnacle that will award a win in case of injury I think Ferrer is definitely the play here.