Suddenly Arsenal look out of their depth and the reasons are not hard to fathom. The relative shallowness of Arsene Wenger's squad was revealed in Saturday's 3-0 defeat at Derby. Already his team's pursuit of Manchester United at the top of the Premiership is beginning to flag.

Little more than a fortnight ago United's imminent visit to Highbury was being regarded as a confrontation between the champions in situ and the champions-in-waiting. Now next Sunday's match is more likely to pose urgent questions about Arsenal's ability to keep up the chase having fallen four points behind.

Such a hypothesis may prove to be an over-reaction. After all this was Arsenal's first league defeat of the season and the first time they had been beaten by a three-goal margin in any competition for more than two and a half years.

Yet doubts about the team's credentials were being expressed before the Derby game and until recently the continued excellence of Dennis Bergkamp made it easy to ignore Arsenal's flaws. In fact, before he started a three-match suspension at Pride Park the Dutchman's form had begun to dip, and it was showing in the overall level of performance.

The scoreless draws with Crystal Palace and Aston Villa suggested that when Bergkamp was unable to shake off tight marking and move through the usual channels, Arsenal's attack would stall. His absence against Derby confirmed Wenger's lack of a plausible alternative strategy and in particular his limitations in midfield.

At various times during the game Nicolas Anelka, Luis Boa Morte, and Christopher Wreh offered French, Portuguese and Liberian assistance. But once Ian Wright had hit the bar with a penalty, Lee Carsley having brought down Patrick Vieira, few chances were created and Arsenal went home with the worrying knowledge that Manchester United have now scored 13 times in two matches while resting, in turn, Teddy Sheringham and Ryan Giggs.

Bergkamp is due to resume his Premier League duties against Liverpool at the end of the month. Yet Wenger may find that it will take more than the Dutchman's return to re-ignite Highbury's championship challenge, for at Derby the defence was as much at fault as the attack. Earlier in the season slipshod work at the back had threatened to undo Bergkamp's good work. On Saturday Arsenal failed their Wanchope test.

They were hardly the first. A fortnight before, on the same ground, Paulo Wanchope had scored twice against Manchester United and made a monkey out of Gary Neville; now the Costa Rican striker repeated the feat and left Arsenal's defenders to share simian similarities.

The difference lay in the way Derby's opponents responded. United had changed their shape, kept the ball and forced a 2-2 draw. On Saturday, it was Derby's change of tack in making Gary Rowett a third centre-back and pushing Robert Kozluk and Chris Powell forward on the flanks which broke the grip Vieira, David Platt and Emmanuel Petit had held on the game before halftime.

Arsenal were also undone by the speed of Dean Sturridge, the tenacity in midfield of Darryl Powell and the clever touches of Francesco Baiano, with his ability to fashion telling first-time passes from balls bouncing at a variety of heights and angles. But above all it is Wanchope who continues to stimulate Derby's progress.

"He has the potential to be a player," said Wenger with the guarded air of one who still could not quite believe his eyes. Even Wanchope's own manager, Jim Smith, had to admit that "as well as being skilful he's sometimes very lucky".

In football, however, luck rarely occurs without some prompting. While Wanchope's opening goal, scored 55 seconds after half-time, owed something to the deflection it took off Steve Bould's heel, making David Seaman's dive appear even later than it was, the Costa Rican's reputation for the unorthodox was equally responsible as he advanced towards a retreating defence. "Even the senior players in the Arsenal team were unsure what to do about him," said Smith.

Derby's second goal, scored just past the hour, followed Sturridge's evasion of a lunge by Tony Adams near the halfway line. Nigel Winterburn intercepted the striker's low centre but instead of clearing the ball with his weaker right foot, he tried to stab it away with his left and merely set up a simple chance for Wanchope.

Platt's well-struck volley was kept out by Martin Poom. Then with nine minutes remaining, Matt Carbon gathered a loose ball which had hit a corner flag and stopped on the Derby byline, and from this innocuous situation another goal followed. Baiano laid off Carbon's pass to Sturridge, who outpaced Petit and chipped the advancing Seaman.

"It would be too easy to excuse this defeat because Bergkamp was not playing," said Wenger. "The problem is more collective than individual."

With Petit starting a suspension next Sunday and Bould facing a three-match ban after receiving his fifth yellow card of the season for a foul on Sturridge, Arsenal surely will need to collect their thoughts over the coming week. Now would be a good time for Wright to rediscover his scoring touch.

Guardian Service. DERBY COUNTY: Poom, Rowett, C.Powell, D Powell, Sturridge (Burton 86), Wanchope, Laursen, Carbon, Carsley, Solis (Kozluk 46), Baiano (Trollope 86). Subs not used: Hoult, Hunt. Booked: Wanchope, Rowett. Goals: Sturridge 82, Wanchope 46, 65.

ARSENAL: Seaman, Dixon, Winterburn (Wreh 69), Vieira, Bould, Adams, Platt, Wright, Anelka (Boa Morte 69), Parlour, Petit. Subs not used: Manninger, Grimandi, Hughes. Booked: Bould, Winterburn, Boa Morte.

Referee: P E Alcock (Redhill).