While many would argue that United's poor defensive play this season has been caused by an aging back line, others would argue that an aging back line is better than an injured back line.
United's defense suffered another blow last weekend against Liverpool, with Nemanja Vidic going down with a concussion and Jonny Evans injuring his hamstring, per ESPN. Although they are both expected to be fit to return this Sunday against Tottenham, this incident can be seen as a microcosm of the current season, as it shows the trend of Evans' and Vidic's extensive time on the sidelines.
With the Reds missing these two, as well as Phil Jones, for large chunks of the season, the team has been forced to rely on the offensive play of Robin van Persie, who has a league-leading 17 goals as of the date of publishing. Simply put, if there is a team that can succeed without playing good defense, it would be United, as they also have stars Javier Hernandez and Wayne Rooney to lead their attack.
If Vidic and Evans continue to get injured and United's offense continues to score, unhealthy defense won't necessarily mean a Man City title repeat. After all, according to The Sun, right-back Micah Richards will be out until February.
For the Reds, an unhealthy defense, which they've dealt with all season, will mean more opportunities for younger players. When healthy, Jones and Chris Smalling have shown bags of potential, and as the possible successors of Vidic and Rio Ferdinand, it is very important for them to get playing experience.
A lack of healthy defenders could also open up the possibility of a formation change for Sir Alex Ferguson. He made headlines earlier this season by finally ditching his conventional 4-4-2 to accommodate Shinji Kagawa. Could he opt out of a four-man back line if his best defenders are hurt?
Possible formations that Sir Alex could use, should he decide on a three-man defense, include a 3-4-3 or a 5-3-2. Given the position needs of the former and United's offensive personnel, it seems that a 3-4-3 would be more fitting, as Kagawa and Javier Hernandez could play off of Robin van Persie on the wings, Rooney could control the field slightly behind the forwards, and Paul Scholes could pass and provide defensive help from the defensive midfield spot.
However, if the defensive injuries worsen and Sir Alex decides to stick to his guns, keeping the same 4-2-3-1 formation in place, it is unlikely that he'll be starting reserves in the back just so that he can keep players in their natural roles. Instead, he'd probably be prompted to move some midfielders to the back, such as Michael Carrick or Alexander Büttner, who only recently started playing on the wings after being bought as a defender.
With United's offensive prowess and squad depth, as well as Sir Alex's tactical genius—should they need it—the Reds and their fans have no reason to worry about injuries in the defense.
Your move, van Persie.