Let's take a look at what happened the last time that one spot in their rotation improved drastically (and I'm not saying that's what will happen with Johnson).
In 2007, Noah Lowry led the team in wins (14) and Matt Cain—refrain from making political jokes, please—led the starters in ERA (3.65) and strikeouts (163). In 2008, Tim Lincecum led the team in wins (18), ERA (2.62), strikeouts (265), and pretty much everything else, on his way to winning the NL Cy Young award—but if you're reading this post, then you probably already knew that. In 2007, the Giants were 71-91 and finished last in the division. In 2008, they were 72-90, finishing 4th (ahead of the 99-loss Padres and behind the 74-win Rockies).
Lincecum was about as good last year (7-5, 4.00 ERA, 146.2 IP in 24 starts) as Johnson was this year (11-10, 3.91 ERA, 184 IP in 30 starts), if in smaller quantity. But unfortunately for the Giants, Johnson isn't due for any sort of crazy stat improvement.
Barry Zito showed some improvement after the All-Star break—he was 4 and 12 with an ERA of 5.62 in the first half, but improved to 6-5, 4.59 in the second half—and Noah Lowry will probably be back from his injury. I would still start Zito and Lowry in the 4 and 5 spots, though, simply because of Lowry's recent injury and the fact that although Zito was better after the All-Star break last year, his performance was worse this year.
So here's my rotation for the Giants in '09:
- RHP Tim Lincecum
- LHP Randy Johnson
- RHP Matt Cain
- LHP Noah Lowry
- LHP Barry Zito
Cain is one of the more interesting pitchers on the list. He lost 14 games last year, but gave up 3 or fewer earned runs in 6 of those games, and he gave up 2 or fewer earner runs in 3 of his losses. If, by some strange miracle, the Giants' offense had managed to scrape together the extra few runs in each of those three starts, he would likely have gone 11-11, with marginally better peripherals (ERA and SO) than Johnson had on the offensively capable Diamondbacks.