Lourdes Gurriel Jr.
An argument has already been made for Lourdes Gurriel Jr. as a signing the Red Sox could make to try to get back, even in small part, at the Yankees for signing Juan Soto. Juan Soto is Juan Soto, and pretty much anything short of the Red Sox getting Yoshinobu Yamamoto wouldn't be a major punch back, but while Soto is obviously an offensive marvel, he's less amazing in the outfield. In fact, Gurriel's defensive numbers on the whole are much more favorable than Soto's — Gurriel ranks in Baseball Savant's 69th percentile, while Soto is in its 9th percentile. And it's not that Gurriel's bat is even significantly worse than Soto's: they put up relatively comparable hit, doubles, and home run numbers this year.
If money was a concern in trading for Tyler O'Neill rather than going after a free agent, that's certainly what made Gurriel fall short in the Red Sox's eyes. He had an excellent postseason with Diamondbacks, which is likely to have driven up his stock this offseason. It's hard to understand why Boston passed on so many good options to go with maybe the most middle-of-the-road candidate out there, but it's unlikely we'll get answers. All we can do is think about a world in which the Red Sox were aggressive and competitive about their offseason signings this year.