I've been thinking Ben Revere.
I don't really see Revere as much of an upgrade if any. Fowler's been worth 89 wRC+ and 0.7 fWAR. Revere is at 98 and 1.6 respectively. If you buy into the concept of WAR in general that suggests he's been worth roughly 1 win over half a season or so. And the thing is this is the upside on Revere. There's not much if any more. Fowler has played pretty bad lately but the point here is that even with him playing bad Revere's only been worth about 10% more as a hitter. I suppose you could argue that as a WC team right now that 1 game over the next half of the season might matter but I don't think you make a trade with the hopes of making a wild card. You trade to win deeper in the post season.
I've said it else where but to me that's not really fixing the problem. Parra similarly has his own issues too as a name that's been thrown around. He's been worth 0.6 fWAR largely for crap defense. He has a 125 wRC+ though. To me, trading for either with the intent of replacing Fowler would be making a move for the sake of making a move because it's hard to argue it significantly makes the team better. And while you can suggest a marginal improvement is still an improvement I'll continue to go back to what I've said about Fowler. He had a wRC+ of 127 in April, last year it was 124 and for his career its 105. This is the very definition of a career year for Parra as his previous best was 106 wRC+. Likewise, this is Revere's highest in his career.
So, if you don't buy Fowler's 2014 season, you're still talking about his career rates being better than all but this half season for Parra. Fowler's 40 points off his career rates vs righties. As such, it's not that he can't hit them he's just not doing it this year for whatever reason. He's 50 points off last years rates vs righties so clearly he's done it recently not to mention I'm guessing his april marks vs righties were decent as well.
Zips has Fowler at .254/.350/.392 the rest of the way(108 wRC+ 0.9 fWAR). Streamer has him at .248/.341/.379(101 wRC+ 0.5 fWAR). And Depth Charts has him at .251/.346/.385(104 wRC+ 0.6 fWAR). I'd argue these are fairly conservative estimates as well considering he's a career .267/.361/.415 hitter. On the contrary, the three places have Revere at 0.8, 0.2 and 0.4 fWAR and none of them have him over 100 wRC+. For Parra, only Zips has him above 100 wRC+ at 104 and they have him at 1, 0.8 and 0.8 fWAR.