If there has been one area that has left Manchester United fans feeling nervous, it's the team's back four this season. And with Rio Ferdinand and Nemanja Vidic on the wrong side of 30, you couldn't blame Sir Alex Ferguson for exploring his options on the market at the centre back position.

If you believe the rumors, that's exactly what he's doing. From Steve Bates of the Mirror:

Although Old Trafford boss Sir Alex Ferguson has concentrated on signing £15 million winger Wilfried Zaha from Crystal Palace in the January transfer window, Benfica star Ezequiel Garay and Torino's Angelo Ogbonna are also on his radar.

The pair are regarded as two of Europe's top talents—and sources close to United say they have been under close scrutiny this season as canny Fergie plans for the future.

Ferdinand, 34, hasn't yet committed his future to United as he ponders the prospect of a one-year deal at the end of the season. And with concerns over the durability of skipper Vidic, Ferguson has asked his scouts to identify Europe's top central defenders.

There you have it.

It's doubtful, of course, that either one of these players would be Manchester-bound in January. These would represent pretty major signings, and seem like the sort of deals more likely to be completed in the summer.

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But United would be wise indeed to consider adding each.

While the team has been fun to watch this season, scoring a league-leading 57 goals in EPL play this year, it has also given up 30 goals, more than Manchester City, Chelsea, Tottenham, Everton, Arsenal, Liverpool, Swansea City and Stoke City.

It's fun to watch United play an open, attacking style—and my goodness, the team is devastating going forward—but the defending hasn't been good for the fanbase's collective blood pressure this season.

If Ferdinand departs or Vidic is deemed to unreliable given prior injury concerns, United must make solidifying the centre back position its top priority this summer. Heck, Patrice Evra won't be able to play forever—perhaps considering some left-back prospects should be in the cards as well.

I know, I know, bring on Leighton Baines. Of course, he won't be easy to pry away from Everton.

In the end, the United faithful should view these rumors in a positive light if they believe them to be true. No one is ever going to question this team's ability to score goals.

But keeping the other team from scoring? Well, that's a problem the Red Devils haven't totally solved just yet.