I think you have to be patient with him, whenever he breaks out of his funk he's going to be an asset to your team. His two year average is 88.5 runs, 14 HR's, 68.5 RBI's, 10 SB's, .283 avg - you won't find that on waivers and I haven't read nor seen anything to indicate he's hiding an injury. It's not like this guy was an end-game guy draft day, he was likely a middle round pick you (correctly) assumed would be play everyday barring injury.
Don't think I'm writing this as an owner without lineup issues, I have them. A lot of them in the infield actually. Right now I have Chad Tracy at 1B (lost his job), Clint Barmes at 2B (like Johnson, also sharing the workload but at least he's not sucking), Jose Lopez at MI (playing, but not playing well), Marco Scutaro at CI (not the long term solution anywhere outside of MI, thankfully he's off to the hot start), JJ Hardy at SS (looks like he's digging his way out of his funk), Mark Reynolds at 3B (yay, stability!) with Kelly Johnson (sucking and in a platoon) and Kevin Kouzmanoff (off to his usual awful start) on the pine. Tracy's the obvious casualty (adding Garko for the next two days, hopefully getting Overbay thereafter and subbing in Lopez at 1B on days when Toronto is up against lefties), but I have so many holes to fill I would need to drop someone else too in order to field a full lineup everyday. I'm not going to do that because I think eventually Johnson's going to be the everyday guy I bought in March, whoever I replace him with won't much the projections from above.
Trust two years of data, not one bad month.