Nick Johnson when healthy is the best offensive player the Nationals have. Period. Better than Zim. Better than Dukes. Someday maybe one of those guys will take another step and overtake him, but Nick Johnson, with his tremendous on-base skills and good power, not to mention solid defense and baserunning, is a very, very good, almost great player.
The injuries indeed are frustrating. It's possible that Nick will never again put together a season like 2006, when his OPS+ of 149 (meaning he was 49% more productive than the average major-league hitter) made him the fourth most valuable first-baseman in all of baseball, behind Albert Pujols, Ryan Howard, and Lance Berkman, and ahead of, among others, Mark Teixeira. In fact, Tex's career-best OPS+ is a nearly identical 151.
Then again, at age 30, Nick Johnson could very well go out and replicate that performance for the next 2-3 seasons. I know, I know. You're rolling your eyes at the thought of Nick even staying on the field for half the next three years. But the point is that his high-end value is far above anything we could hope to get for him now, and his trade value if he, for instance, has a true Johnson-esque first half in 2009, will skyrocket.
I could have been supportive of trading Nick at his peak value. In 2005 or 2006, he could have fetched a king's ransom from a contender in need of a first-baseman. But to trade him now would be to flip him at his absolute lowest value, and it just doesn't make sense.
We're far better off just holding onto him, hoping that he rebounds, and then if we're committed to flipping him for youth (not a terrible idea), making a deal after he re-establishes himself as an elite offensive player.
Or, if you're more inclined to focues on making the Nationals better in the short-term, then we should definitely hold onto him, because no one we're going to get back for him has the potential to be what Nick could be (all together now...) if healthy.
As for the Oakland A's reported interest in Nick, every Nationals fan should shudder at the thought of Billy Beane dealing with Jim Bowden. The chances that we come out on the better end of a deal between those two--arguably the very best and very worst GMs in all of baseball--is remote indeed.