Beni's not hitting with authority and his body language, like throwing his bat on the ground powerafter striking out seem to indicate he is very frustrated with himself this year. He is still young and is not yet paid like he will be in future, but to put a realistic value on him we have to look at his overall game.
Defensively he is a good but not great outfielder with good speed and a good arm. Otherwise, we could say he is a good but not great defensive outfielder but he can play LF at Fenway effectively. As far as his hitting is concerned, Beni doesn't pull the ball that often nor does he have much HR power. Since he isn't a power hitter, to be of special value to us as a hitter, we need him to have a solid on base percentage but this year he has a lot of swings and misses and his BA is down and falling.
My take on a young homegrown player is that he may improve his OBP but is unlikely to ever deliver much power. As a result, he may be a player we can use for trade bait and replace him with someone at least as good. If our fortunes don't suggest 2019 will be a playoff year, it is a place to look to improve for 2020.