According to RARP - a Baseball Prospectus statistic measuring offensive runs above replacement by position - Raul Ibanez has been worth 6.1 runs above a replacement-level left fielder with the bat so far this year.
If you assume that a replacement-level offensive left fielder would be about average with the glove (a true replacement-level hitter and fielder would never make the Majors), then that allows us to use UZR and add it to RARP to get an approximate total player value. Based on UZR numbers from about four weeks ago, Ibanez is horrible in the field, to the tune of being about 19 runs below average to date.
So combined, Raul has been worth roughly 13 runs below a replacement-level left fielder this season. The average among all regular LF's so far has been +15. Raul's right at the bottom, tied with Craig Monroe and ahead of only Jay Gibbons. Everyone else - Jay Payton, Adam Lind, Jason Michaels, Emil Brown; everyone - has been better.
For Raul Ibanez to compensate for his defense and be a replacement-level left fielder, he'd have to hit like Luis Gonzalez or Matt Diaz. For Raul Ibanez to compensate for his defense and be an average left fielder, he'd have to hit like Carlos Lee or Adam Dunn. Instead, he's hit like Rob Mackowiak.
More importantly, if we say that Adam Jones is roughly an average defensive left fielder, the only way that he wouldn't represent an upgrade is if he hit worse than any other left fielder in the league. Worse than Monroe (.220/.264/.375), worse than Gibbons (.229/.274/.350), worse than everyone. And if you agree with me in thinking that he'd make for an above-average defensive LF, then even if he absolutely, totally tanked at the plate, he'd still be no worse than a lateral move.
This is what I mean when I say that there's no possible way for Adam Jones to be a downgrade. There really is no possible way for Adam Jones to be a downgrade. Not only would he have to be a letdown with the glove, but he'd also have to be one of the worst hitters we've ever seen, and I just don't see any way that both of those things happen.
The absolute best-case scenario for Raul Ibanez going forward is that he returns to his 2006 level of offensive production while his defense regresses (progresses?) to the mean. Using a .2/.3/.5 weighting system for his last three seasons in the field, both of these things combined would make Raul a +6 run (over replacement LF) player for the rest of the year, or ~+19 over a full season. For an average defensive Adam Jones to match that, he'd have to hit like Jason Bay (.257/.332/.433). For an above-average defensive Adam Jones to match that, he'd have to hit like Mackowiak (.277/.348/.404) or, say, this year's Raul Ibanez (.259/.316/.419). These tasks shouldn't be difficult for him to accomplish.
Adam Jones is the second-best outfielder in the system, and since being promoted from AAA a month too late, according to the manager he's pretty much going to be given the Jason Ellison role of pinch-running and playing the field for the ninth. If the Mariners end up missing the playoffs by only a few games, they have no one to blame but themselves.