Holt speaks of his decline after suffering the concussion and his inability to feel right until finding a new medication, which he is now taking daily. He says he feels much more normal now and that his spring training performance is reflecting it.
I tend to agree with your assessent of Brock. That his versatility is not of as much value to us now that we have a DH who can play in the outfield, Hanley who can play 1st base and Nunez who is flexible and will be used in utility once Pedey returns. We also have Swihart who is going to make the roster and is quite versatile. That leaves Holt fighting for the one remaining roster spot with Marrero and that spot may also dry up when Pedey returns.
As you point out, Brock is not the number 1 option in utility at any one position. If he regains his hitting stroke, as he believes he has, he could return to a 280 hitter with not much power but there is the question of whether his health will hold up on the long term. Brock has been a nice player and a good guy for the Sox, so it will be interesting to see how he is handled. Will the Sox look ahead to cover the inevitable injury bug, where one or more of our players gets laid up for 15 to 60 days? It appears that Marrero or Holt will be shopped and traded if possible.