According to the Boston Herald's Jeff Howe, Dobson actually had a permanent screw inserted into his left foot during the March surgery to repair a stress fracture.
The screw speaks to the severity of the injury and invasiveness of the surgery. It also suggests that the initial estimation of a two-to-three month recovery was a bit optimistic.
According to Howe, Dobson didn't even begin running until July, which means he's fairly early in the progression that would lead to him planting and cutting at full speed without pain. It also suggests his conditioning will need to be monitored when he is able to return.
Since stress fractures generally have a high recurrence risk -- Dobson was injured twice his rookie season -- the Patriots medical staff is taking it slowly with the receiver. The team has no timetable for when Dobson will return from the PUP list, per Howe.
When Dobson's healthy, his speed provides the Patriots' offense with an element it's otherwise lacking. At this point, however, his foot is looking like it could be a long-term worry.