Tampa's receiving corp was anemic last year with Evans being the only reliable pass catcher (at WR - Brate has developed nicely into a solid pass catching TE). Adding D. Jax is huge to the offense from an NFL perspective. He will open up the field significantly more than what the Bucs have had in the past. They won't need him to be a super star, but if he's the kind of guy he's been most of his career (60 rec / 1100 yards / 6 TD's - or there abouts), he'll improve the offense immensely. From a fantasy perspective, that should put him at the cusp of a #2/#3 WR, but NFL-wise, he will help across the board. The running lanes should be more open as no one will be able to put 8 in the box because there will be 2 WR's on the field requiring double coverage.
In the end, it will all come down to Winston's progression. He still has things that need to be improved. If he doesn't refine his issues, Jackson's impact will not be as hoped.
I would be surprised if they bring V-Jax back. They like Adam Humphries and so do a lot of fans. He's an excellent #4 and decent #3 right now who could really be a better player with DeSean being on the field. If the Bucs stand pat on WR at draft time, they'll be in pretty good shape, but I do think they'll look to add a WR somewhere between rounds 2 and 4, pushing Humphries to the #4 role. At that point, there's no really reason to bring VJ back.