What do they need to upgrade?

3B and pitching (always)

Who do you think they will get?

Professional 3B rental on cheap dollars imo.

Will they stay under the cap?

Maybe? Though DD says they don't have to. https://www.mlbtraderumors.com/2017/...t-cabrera.html

The Red Sox do not need to stay under the luxury tax line in making their deadline moves, president of baseball operations Dave Dombrowski said in an appearance today on WEEI. Recent estimates put Boston about $9MM shy of the threshold, which has added implications under the new CBA since the tax escalates when it is owed in consecutive seasons. While the Sox will no doubt weigh that factor in assessing possible trades, it’s notable that the organization doesn’t feel compelled to stay within those limits.