Henry must follow Van Nistelrooy's lead and add a killer touch to that brilliance

Long before the end of their second straight FA Cup final victory you could see more clearly than ever before that the real question is not how good are Arsenal but how great is Thierry Henry.

The first question can only be answered in the light of the second. This is because Henry is more than a vital element in his team. He is Arsenal.

If Patrick Vieira is the piston, Henry is the soul. He draws the boundaries of the team performance. He is their strength and their weakness, their light and their shadow, and if there had been a little more of the light and less of the shadow Southampton would probably have needed to be hooked up for a massive transfusion of untwisted blood.

When Henry soars, so do Arsenal. When he wilts, so do Arsenal.

He threatened a riot of brilliance, as he always does, but on this occasion, as on several others in the title run-in, his suit of light, while shining as brightly as ever, was not accompanied by a matador's sword. And is this not the story of an Arsenal season that, even after their extravagant celebration of winning a trophy that Arsène Wenger long-ago downgraded to the foot of big-time football's totem pole, remains largely misbegotten? Arsenal's retaining of one half of last season's Double restored some basic pride, but it surely intensified rather than resolved that big question. No doubt Wenger was right to insist: "This group has a future... We showed I have some boys I can rely on," but that was never in doubt.

The issue is the extent of that future, and it was plain enough in the Millennium Stadium that for all Southampton's adherence to their manager Gordon Strachan's demand for unceasing "gut-running" a definitive answer would not, and could not, be forthcoming.

That will have to await Henry's handling of the three more Premiership campaigns guaranteed under a new contract – and evidence that, like Ruud Van Nistelrooy, he can dig down into his resources and come up with a new dimension.

That is what the big Dutchman did to make himself the Premiership's man of destiny this last season – and what Henry did not. Of course Henry made a brilliant contribution to Arsenal's latest challenge to Manchester United's deeply entrenched status as the team everyone has to beat, and in Cardiff we saw still more evidence of his uniquely graceful virtuosity.

But we also saw the flaw in the diamond. We saw bewildering pace and sublime control – and critical irresolution. Three times he could have killed off Southampton but in the last desperate moments the difference between the teams was not Henry's talent but Robert Pires' goal and a spectacular save by the time-expired David Seaman. Ashley Cole also staved off a potentially sickening re-enactment of the collapse which may have cost the Premiership at Bolton.

Great teams – and great players – produce more decisive work. They cut down lesser troops. They do not work the clock. They work their greatness. It is true that sometimes they cheat, as Maradona did in the Azteca stadium in 1986, but generally not when they have a better option, as Henry did when he bore down on a thoroughly intimidated defence only to produce not devastation but a dive.

This, minus the dive, was the story of Henry here two years ago when he produced some dream-like play and Michael Owen delivered the killer blows. It was the tale of the Premiership debacle. While Van Nistelrooy waxed, Henry waned. Not in the thrilling delicacy of his art, but in the level of his execution. So if the silver of the Cup was won, the gold of greatness had to be withheld.

That, surely, applies both to the player and his team. When Wenger says that his team suffered more a stumble than a crisis of belief the suspicion has to be that he is whistling past the graveyard of his hopes. Yes, Henry would grace a Manchester United team, but would he necessarily improve it? And if Arsenal brought a red glow to the sky over England in the first half of the season, how many of their players would automatically walk into the United side. Vieira, no doubt. Campbell, perhaps. Henry replacing Van Nistelrooy? Or playing alongside him, but at the expense of Scholes? Given Sir Alex Ferguson's habit of attaching as much weight to heart as talent, perhaps not.

It means that if Wenger is to effectively resume the battle later this year, he needs three successful signings: a goalkeeper to replace a David Seaman, whose brilliant save from Brett Ormerod could not wipe away some alarming indecision when the ball was in the air, and two central defenders. Most of all, though, he needs Henry to make one last leap, It would be the final stage of an extraordinary development. When he came from France he was quick but marginal, Now he is quick, mesmerising and utterly central to the Arsenal effort – and any appraisal of their potential to achieve genuine greatness. The third stage is not the icing on the cake but the snow on the mountain peak. You do not learn greatness but you can come to it with the help of a little self-analysis.

Henry, surely, has the strength of mind to think through his situation. At times his expression says that for all his speed and artistry he is incomplete. His effectiveness would leap if he headed the ball better – and more frequently got to that most hurtful place, the far post. Still in his mid-twenties, Henry has the time and the incentive to grow in the way of Van Nistelrooy when United bulldozed their way to the title. Van Nistelrooy was not always so aggressive. The Dutchman may not have learned aggression, maybe it is beyond the power of instruction, but in the end it flowed from the extent of his commitment and ambition.

Maybe the reality of the Millennium Stadium will prompt a pull of the trigger by Henry. The truth was not that Arsenal had reclaimed the ground they held before the the fading of their Premiership challenge. It was that finally they had to struggle to impose hugely superior talent. Worse still, Henry was booked – legitimately – for diving. He can do better. For his own greatness – and Arsenal's – he must.