As a 4th outfielder, Carrera isn't that bad, especially considering what he is paid.
Short of paying for a (close to) guaranteed upgrade I've no issue with keeping him as a 4th outfielder until one or more of our prospects takes his place. To get someone who's likely to be better, we'd have to pay a lot more than what Carrera is paid. Just look at what Reddick got from the Astros - 4 years $52m projected for 1.5 WAR, if everything goes right he might give you up to 3 WAR. Solid but hardly a star.
The main issue is that he plays too often when we suffer from injuries, like this season.