Trading Konerko for Erstad involves risks taken by both teams, which therefore, makes it an equitable deal. Sox risk: Will Erstad be able to stay healthy. Angels risk: Will Konerko rebound to the hitter he has been in the past.
The point of this deal is that Konerko fits the Angels needs and Erstad fits ours. You swap salaries and risks, but if either works out, someones needs get met. If you don't make this deal, neither team's needs are addressed.