It is hard to get excited about Phillips. He didn't play particularly well for the Angels last season. His OPS+ has been under 100 for several seasons in a row. In fact, you have to go back to 2011 to find a season in which his OPS+ was above 100. His defense has declined. He is 37 and so he isn't getting better--he is getting worse. He doesn't have exceptional splits against lefties (over the last 3 seasons), thus a Holt/Phillips platoon doesn't seem like a good idea with Phillips playing exclusively against lefties.
True, Phillips is probably better than Nunez, but that is because Nunez is a complete train wreck.
Holt is better than Phillips and Nunez, but I would look to upgrade the offense and save Holt for that utility role. I would trade for Cabrera. The difference between Cabrera, on the one hand, and Nunez and Phillips, on the other, is night and day. Over the last four seasons, Cabrera's OPS+ has been over 100. He is 32 years old--still young enough. Cabrera would give the Red Sox some extra offense on those days when Betts and Martinez are not carrying the club. Cabrera would be a high priority upgrade; not only should the Red Sox trade for him--they should resign him in the offseason.
It is tempting to release Nunez, he has certainly earned it, but he has 4 million coming to him next year and can play several different positions (albeit not that well). Nunez goes to the bench -- no more starts for Nunez.
The key isn't to give Holt more or less playing time, the key is to demote Nunez to the bench, trade for Cabrera, and play Holt at various positions, giving the starters a day of rest and keeping them fresh.
The Mets have two players the Red Sox could use--Wheeler and Cabrera. Get it done, Dave.