One of the great pieces of marketing in modern football is the holding midfielder.
The once-fashionable 3-5-2 formation was killed off as a credible choice when people pointed out that you only need three centre-backs if you don't have two who can do the job.
No manager wanted to admit that they couldn't sign two central defenders who were good enough, but that didn't magically create enough to go around; the preference for three men wasn't unjustified previously because there simply weren't enough top quality centre-backs for each team to have just two.
There still aren't, which is why these days managers play two men in the centre of defence, and a third, less capable defensively, but with at least some attacking ability, and call him a holding or defensive midfielder.
It got the fans off the managers' backs, but holding midfielders stay back the whole time, and go to wherever in the defense needs reinforcing. That makes telling the difference between them and a third centre-back nearly impossible, since there is virtually none.
Arsenal, it goes without saying, have never played any such system under Arsene Wenger. Granted, the way Arsene's full-backs get forwards is similar to the way wing-backs play in a 3-5-2, but all that means is that Arsenal play two formations: a 2-4-4 when they have the ball, and a 4-4-2 without it.
The much-mentioned Patrick Vieira is a case in point. Although he's generally spoken of as if he spent his whole time winning the ball and protecting the defense, he was nothing like that limited. Players who can only defend are commonplace, and we call them 'defenders'.
They generally aren't even good enough to play in the Premiership, if that's the only skill they have, but Vieira was far more than that. Where he excelled was that he was a top quality defender, allied with an Arsenal-attacking-quality midfielder.
The much-lamented Flamster, on the other hand, couldn't pass (better than Gallas), dribble, cross, or shoot. This makes him what is known as 'a defender', and he isn't good enough at defending to be a centre-back or full-back. He's good but nothing unusual, which is why his wage demands weren't worth meeting.
Dennis Nilsen was a serial killer—15 victims—first in Willesden, and then later in Muswell Hill. Denilson is a different person (N.B., the currently confused commentators), what with being a rather talented midfielder, who's also developing a mean streak in defence. He physically resembles Flamini, but in playing style, he's far closer to Vieira. He's quietly good (and improving) defensively, and has a range of passing that's top quality.
Unless they've just woken from a 20-year coma, no-one's going to disagree that Arsenal are an attacking team. An attacking team can't afford the third-defender sort of midfielder, so to improve the defense the only option is to get a better centre-back.