In Tuesday's win on penalties against Inter, the United defensive setup was put under the microscope, and the back three trotted out by manager Louis van Gaal held the Italian side to just one shot on goal.
Van Gaal opted to start the trio of Phil Jones, Chris Smalling and Jonny Evans in the back, with Ashley Young and Antonio Valencia being utilized as wing-backs.
Young, who was only one of two United players to play the entire 90 minutes, switched over to the right after the half.
"I started on the left then ended on the right. I have the confidence in myself to go out and play well," Young said.
Throughout most of the half, the trio in the back—which spent most of the match close to midfield—watched their teammates keep possession in search of chances on Samir Handanovic's goal.
Smalling was the player who stood out due to his communication from the middle of defense.
When United were moving forward, Smalling was the eyes of the team as he shouted out where players like Juan Mata and Ander Herrera should pass the ball.
In the back three, the man in the middle is relied on more because he can see the entire field from a central position, while the other two centre-backs are employed with a side of the pitch to defend.
The entire defense did their job well, with Inter putting little pressure on both Anders Lindegaard and David de Gea.
"When you see our three matches, the first match was zero goals. The second it was a fantastic goal from [Roma's Miralem] Pjanic and a penalty against that was given by the referee. Today again zero goals," Van Gaal said.
For the most part, Evans and Jones held their ground, but on one occasion in the middle of the first half, Evans sprinted over to help Jones contain the Inter attack.
That plan worked out well, but it did leave Young alone on the left side of the box to defend a potential cross.
On another day, the situation may have had a different result. Since Inter lacked an impetus in attack for most of the game, though, the Red Devils got off easy on that play.
The main concern that the 3-4-1-2 system brings is how wingers like Young and Valencia will defend after making surging runs forward.
Neither player is a natural defender, but if they want to retain playing time under Van Gaal, they must adjust to the new manager's style of play.
The wingers aren't the only ones being analyzed by the Dutch manager, as the back three are not set in stone yet.
Van Gaal admitted in his press conference after the game that the club are still looking for players to fill the voids left by older players Rio Ferdinand, who left for Queens Park Rangers, and Nemanja Vidic, who started for Inter at FedEx Field.
"We are looking for defenders because a lot of defenders have gone. We have to replace them," Van Gaal said.
One player the new United boss will not be able to bring in is Stefan de Vrij. According to Mike Keegan of BBC Sport, he signed with Lazio. De Vrij was a major part of the Netherlands' success at the World Cup, and it would have benefited the Red Devils greatly had he joined the club.
Before the potential replacements can be added, Van Gaal has a chance to look at young defenders Michael Keane and Tyler Blackett to see how they fit into his plans—if they do at all.
"I like also Michael Keane and Tyler Blackett. We have to wait and see if we can [defend] in more matches like we did today," Van Gaal said.
United may be deep into their preparation for the upcoming English Premier League season, but the team we have seen during the International Champions Cup may not be the side we see against Swansea City in the season opener August 16—especially if Van Gaal doesn't like what he has on defense.
Follow Joe on Twitter, @JTansey90.
All quotes obtained firsthand unless otherwise noted.