Dave Roberts to the Red Sox
Hey, look, Dave Roberts made a list and it doesn't involve his questionable in-game management decisions! Roberts' time as a player with the Dodgers wasn't particularly special, as he posted a .718 OPS season in 2002 followed by a .638 mark in 2003. However, the one thing he could do is steal bases -- he had 121 steals in 2.5 seasons of work with the Dodgers. While his ability to get on base wasn't exactly what you would want in the leadoff spot, he did cause havoc when he reached safely and that had some real value.
Trading Roberts in 2004 wasn't a terrible decision per se because, again, his value was somewhat limited. However, the return here is the problem, as the Dodgers got only outfield prospect Henri Stanley from the Red Sox. Roberts would win a World Series with Boston that year and put up some decent seasons with the Padres and Giants afterward. As for Stanley, he never made it past Triple-A and was done playing in the minors after 2005.
Don Newcombe to the Reds
There were several trades from way back that are strong contenders for this list, but we wanted to make sure to highlight when LA decided to part with legend Don Newcombe. Newcombe was just a couple of seasons removed from winning both the MVP and Cy Young Award in 1956 when the team dealt him to Cincinnati in 1958 for Steve Bilko and Johnny Klippstein.
Not only was trading away one of the franchise's few remaining stars from their days in Brooklyn not a great look (even though Newcombe was declining), Newcombe was still a useful pitcher until he retired in 1960. As for who the Dodgers got in return, Bilko would only play 47 games for the Dodgers and wasn't very good when he did play. Klippstein was decent out of the bullpen immediately after the trade, but then proceeded to post a 5.91 ERA in 28 appearances in 1959 before he got shipped off to the then-Indians.
Ron Cey to the Cubs
If you look back at most of the pre-2000's deals that involve a team trading a guy with multiple All-Star Game appearances, the returns are almost always lackluster. As it turns out, a lot of general managers were not good at getting real value for trades before analytics changed the game. That was certainly the case when the Dodgers traded six-time All-Star Ron Cey to the Cubs for Vance Lovelace and Dan Cataline.
Now, sure, Cey was not at his peak when that trade went down back in 1983, but he was still a very productive hitter who knew how to draw walks and had some real pop. Moreover, he remained productive for another four-ish seasons with the Cubs before finishing his career with the Athletics and notching three 20+ home run seasons. As for Lovelace and Cataline, they didn't amount to much on the field with most of their careers spent toiling in the minors.
John Franco to the Reds
John Franco is best known for his time with the Mets where he put together an impressive amount of saves and a long, storied career. However, he actually got his start with the Dodgers and was traded to the Reds in 1983 in a package to acquire a bench player, Rafael Landestoy.
In fairness to LA here, this one only looks bad in hindsight. Landestoy was actually one of the better bench guys available and Franco was not a prized prospect the vast majority of folks' minds. Most minor-league relievers that get traded for help at the big league level don't pan out. Unfortunately for the Dodgers, Landestoy would retire one season later and Franco would go on to save 424 games over a 21-year career. Baseball is funny, sometimes.