In recent years, full-backs have become increasingly key positions on a football field, developing into ever more prominent weapons in the final third.
Their influence going forward is so often behind the final cut-back or decisive two-on-one that leads to a goal, meaning there's a growing emphasis on the attacking attributes of these players.
As long as a full-back is contributing in the final third, uncertainly at the back is often glossed over, or at least persevered with.
But how do they measure up against each other, statistically?
First off, here's a look at some numbers to evaluate their input at the back.
The results here are very close, but suggest both players have obvious strengths to their game and are a little more consistent than many imagine.
Overall, Rafael has been involved in a marginally more successful defence, conceding at a slightly lower rate, but neither player emerges as a clear winner.
The Brazilian is keener to get involved in the darker arts of the challenge area, yet while he's more prolific—with his 73 tackles placing him in the top five among defenders—it's Johnson who's the better tackler.
In fact, Liverpool should be looking for Johnson to tackle more, as his success rate is the eighth-best in the league among players with more than 50 tackles. That reflects why he is dribbled past less often than Rafael and has won possession more often in the defensive third.
Johnson is also more involved and more successful in the air, while Rafael has been far more effective clearing the danger and reading the opposition, intercepting and disrupting more attacks.
There is certainly little to choose between the pair here, with both producing very respectable defensive returns.
Next, here's a look at some attacking results to gauge just how influential these players have been going forward.
Again, the numbers reflect favourably on the pair, highlighting two of the Premier League's more attacking full-backs.
Rafael is the more accurate passer, but it's Johnson who's been more involved for his side in attacking terms. In fact, only Everton's Leighton Baines has made more final-third passes among defenders.
From this return, it suggests both players attack via different methods.
Johnson looks to dribble at opponents, before cutting in and either setting up a teammate or shooting at goal himself. Out of defenders, no player has had more shots or has dribbled more often than the Liverpool man and he's also created the fourth most chances. A trio of impressive feats, with the only contradiction being it's Rafael who's found the net more.
The United man adopts an alternative approach. Although he's been more accurate on the far fewer occasions he's dribbled, the Brazilian prefers to overlap and send in a cross, something he's more successful at than Johnson.
These approaches also reflect the style of their side, with Liverpool looking to pass more and construct quick, incisive moves, while United are renowned for their fondness of width.
Johnson's numbers, especially with regards to his creativity and prominence in the final third, show his approach to be slightly more successful.
These statistics feature two of the Premier League's leading, more complete full-backs who have contributed much for their side over the course of the season.
In this current era of full-back's needing an attacking bias, both these players possess all the credentials to be effective going forward, while adding enough solidity at the back.
In attack, Johnson is especially penetrative, and his dribbling, shooting and creative numbers line up favourable among wingers, let alone defenders.
Rafael is less central to his side's approach, but has been consistent in most facets of his game this season. At just 22, six years younger than Johnson, he's still developing and has the potential to become a leading full-back for many years to come.
Comparing the two, defensively they essentially cancel the other out, but in attack, Johnson's greater involvement and significant input in the creative stakes see him shade Rafael, for now.
Statistics via EPL Index