The Ravens needed to fix the offensive line. Trading for a new left tackle is certainly a start.
By acquiring left tackle Eugene Monroe, the Ravens are essentially saying that the offensive line troubles are personnel related instead of an issue with scheme. It also suggests the offensive line's top culprit (or scapegoat) for its problems is with Bryant McKinnie, who would appear to be losing his starting job for the second year in a row.
You don't make this trade at this time of the year for Monroe to back up McKinnie. it's pretty clear who will be starting this Sunday against Miami.
In the NFL, your left tackle is presumably your best offensive lineman. In the run game, the Ravens haven't run much to the left and have had zero success doing so.
The Ravens have run the ball just nine times behind McKinnie in four games. On those nine runs, the Ravens rank dead last at just 1.78 yards per carry behind McKinnie (surprisingly not last on the roster — the Ravens are only getting one yard per carry behind right tackle Michael Oher and 1.65 yards per carry behind right guard Marshal Yanda.)
Now, Monroe hasn't exactly had much success in Jacksonville this year. The Jaguars have only run the ball eight times behind him for an average of 1.88 yards per carry. You can also argue that without a passing game of any sort, the Jaguars' running game is at an extreme disadvantage compared to anyone else in the NFL. The organization has also harbored a culture of losing for such a long time, which players can only take for so long.
Monroe has the ability to fire off the ball when its snapped, a trait the Ravens line has not shown much of this year. Perhaps, his ability to do so will translate over to the rest of the group (particularly at center) and the Ravens line can generate more of a push in the run game.
Monroe was a highly regarded left tackle prospect coming out of Virginia in 2009. He was regarded as one of Jacksonville's best offensive players but never got the deserved attention due to how bad the Jaguars have been in recent years.
This can be viewed as an upgrade from McKinnie. This could be the first step for the Ravens in getting a consistent ground game back on offense.