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This isn't a lesson from the match against Blackburn necessarily, only a point that needs to be made.
Against Blackburn, Tottenham's only two fit center-backs, Younes Kaboul and Sebastien Bassong, were not impenetrable, but they got the job done. Christopher Samba gave them both a bit of trouble in the air, but they both seemed up for the challenge, despite a few scares.
The problem, of course, is that these two are, in fact, Tottenham's only fit center-backs, and if they get injured, Tottenham will likely have to start sticking guys like Jake Livermore and Sandro Ranieri awkwardly in defense. This may work, but it is unlikely to yield good results consistently.
As I mention in my Tottenham transfer article here, Rio Ferdinand and Gary Cahill are two very good candidates for transfer, especially as the pressure continues to mount on Ferdinand at Manchester United and Cahill's Bolton continue to struggle at the bottom end of the table.
Other good center-back targets mentioned in the article include Marc Bartra, currently with Barcelona. Christopher Samba put a fine audition on for Tottenham with Rovers on Sunday, and would also be a very nice target.
With William Gallas seemingly out indefinitely, and Ledley King's status almost always up in the air, a center-back signing is not only favorable, but necessary. Yes, it'll surely cut into Younes Kaboul and definitely Sebastien Bassong's playing time when everyone is fit, but the needs of the team definitely come before the needs of the player in these cases.
And in any case, Bassong is strongly in favor of a move to QPR, and could partially fund the move for a more experienced, reliable center-back, or maybe even two.