Here are his on-base, slugging, and wOBA numbers since 2008:
Markakis's 2013 numbers are included above, but it's still sort of early in the season. Still, besides reversing the power trend last season, his power has been declining pretty steadily. He obviously has plenty of time to turn things around in 2013, but it wouldn't be surprising to see his slugging percentage stay closer to .400 than .471.
It's important that Markakis is still efficient at getting on base (though he is in a funk right now). His walk rate has dipped slightly the last few years, but he's still doing fine in that department (7.3 BB%). It's likely that that number will be over 8 percent before the season is over. But if Markakis is not going to do much more than hit lots of singles and walk (he only has seven extra-base hits at the moment), then the O's are paying a ton of money for a corner outfielder whose defensive skills also appear to be declining.
It's not hard to speculate on a couple of reasons for Markakis's recent lack of power. I'm not going to say the Orioles ran Markakis into the ground ever since he debuted with the team, but O's managers probably could have done a better job of giving him more rest. Before his injury-shortened 2012, he played in at least 147 games in every season -- and at least 157 after 2006. That workload would take a toll on many players, even one as durable as Markakis.
Also, due to some bad luck, Markakis struggled to stay on the field in 2012. Things got off to an ominous start when he had abdominal surgery in January of 2012, but he recovered sufficiently before the season started. Then in the middle of the season, he missed all of June and about half of July after having surgery to remove a broken hamate bone from his right wrist. A couple months later a CC Sabathia fastball broke his left thumb in September, which ended his season. Then in March, it was reported that he was dealing with a sore neck, so he rested for a couple of weeks.
Oddly enough, Markakis just had his best year since 2008. But was it just a temporary improvement? Are the injuries catching up to him now? Really, it's likely a combination of both. At this point in his career, he probably won't be as good offensively going forward as he was last season (and certainly not in 2008). It's also possible that the thumb and neck injuries are having at least some negative effect on his power, or even just his swing in general.
The best thing the O's could do is try to keep Markakis as healthy as possible by giving him some time at DH, like they've already done a few times this season, and hope that he picks it up shortly. His numbers certainly aren't approaching Ryan Flaherty territory, and there's plenty of time for Markakis to go on a tear and have his numbers tick up a bit.
But it would be foolish at this point to expect Markakis to start hitting a bunch of home runs or even 40-plus doubles again. He's still a very smart hitter who can take a walk and slap hits to any part of the field. But those hits are likely going to be singles.