While everyone assumes that given the lack of depth with the forwards that will be the Pittsburgh Penguins biggest trade target, it's important to remember that defensively the team needs to tighten up as well. Paul Martin (broken hand) and Kris Letang (stroke) are out of the lineup. From Rob Rossi of the Trib:
The Penguins, like almost every Cup contender, are interested in MacDonald, an impending free agent with a cap-friendly $550,000 hit. He is perhaps the most sought-after rental player on the trade market.
Upgrading on defense is the Penguins’ second priority after Kesler because they are operating as though Kris Letang (stroke) will not play again until at least next season. They do believe, as of now, that Paul Martin (right hand) will be ready for Round 1 of the Stanley Cup playoffs – but coaches are not at all comfortable with going into the postseason possibly having to play Simon Despres, if he is not traded, or Robert Bortuzzo. Neither player is all that much trusted by the staff at this point.
The Penguins believe they can make the salary-cap work to add Kesler and a veteran defenseman. That is partly because a deal with Kesler would be a hockey move that involves player(s) taking up cap space.
To add a defenseman, the Penguins would consider adding a rental player on an expiring contract.