This is, on the cover, a puzzling trade until you start looking at the caveats.  Texas needed to focus on the bullpen now, especially lefties now that Neal Cotts won't be returning to the Rangers.  However, Ross never could get lefties out; he has no effective pitch to his strong arm.  So, Ross wasn't filling a role in the Texas bullpen despite his left-handedness.  Boston didn't need a lefty, but they did need someone with a proven history of getting out righties.  Ergo, Ross.

Ross sure as heck wasn't going to start again in Texas anytime soon.  It's entirely possible he asked for the trade when the Rangers made it clear he was around the double-digit mark in starting pitching depth.

Boston has been high on Ranaudo, but he has a handful of promising pitchers in front of him and didn't make the most of his big-league debut last year.  Additionally, Ranaudo spent a great deal of his early career injured, so he carries additional risk.

Ranaudo isn't an ace-in-waiting, but if he could up his control of his low-to-mid 90's fast ball, he's got the pitches to be an everyday starter.  He's a good shot right now to make some rotations, and I suspect Texas views him primarily as a potential trade chip, and secondarily as organizational depth if the pitching staff collapses again.