The argument can be made that Jason Kendall, at least through the first 2300 plate appearances of his career, had a similar offensive profile to that of Joe Mauer, but we can not assume Mauer will decline similarly to Kendall just because they are similar offensively. Instead of pointing out that Mauer and Kendall were at least similar at some point offensively and trying to connect things that way, we should be finding out why Kendall dropped off so precipitously.
Remember that gruesome ankle injury Kendall had? I was wondering if that may have started his decline, so I looked it up. Then I got sidetracked into typing this marathon of a post. His only shortened season while with the Pirates was 1999, so that must have been the year it happened. He came back the following season and played 152 games and posted a 124 OPS+, so I don't think we can use the injury as an excuse.
Here are Kendall's ISO power numbers by year while he was with the Pirates:
1996: .101 (22 years old)
1997: .140
1998: .146
1999: .179 (injury shortened but still played 78 games)
2000: .150 (age 26 season)
2001: .092 (note the drop in power - it never returned)
2002: .073
2003: .091
2004: .071 (30 years old)
So Kendall's power disappeared in 2001. At that point, his offensive value hinged solely on his ability to get on base. He still posted a .399 OBP in both 2003 and 2004. Those were the last 2 seasons he posted an above average OPS+ (remember 100 is average). After posting a 107 OPS+ in 2004, his last season with the Pirates, he never posted an OPS+ higher than 88.
Kendall's 162 game averages for his Pirates career from 1996-2004 (5283 plate appearances) were a .306 batting average, 52 Ks, 59 BBs, 182 hits, 9 home runs, 34 doubles, and 4 triples.
Mauer's 162 game averages from 2004-current (4243 plate appearances) are a .324 batting average, 71 Ks, 83 BBs, 194 hits, 15 home runs, 39 doubles, and 3 triples.
Not all that comparable.
But if we look at Kendall's numbers through the 2000 season before his power vanished (2294 plate appearances), Kendall's 162 game averages were a reasonably Mauer-esque .314 batting average, 61 Ks, 63 BBs, 179 hits, 12 home runs, 37 doubles, and 6 triples.
The extra base hits are very similar, but Mauer still easily trumps Kendall in hits and walks.
What to take from all of this? Kendall hit triples way better than Mauer. At least we can use the names Kendall and Mauer in the same sentence while keeping a straight face.
One other thing of interest: Before the injury, Kendall had 71 stolen bases vs. 16 caught stealing. After the injury, Kendall had 58 stolen bases against 41 caught stealing (while still with the Pirates). It may be safe to assume the injury took away his once respectable stolen base ability. Maybe that was the beginning of the end for him. Maybe not. The 124 OPS+ the following season says something else took away his ability to hit.