Weather conditions in Hampshire were perfectly calm on Saturday afternoon but Southampton’s players must still have been braced for an extremely bumpy landing as their plane approached the south coast.

Twenty four hours earlier, they had taken off for the north-east quite possibly believing they had already cracked the Championship. If so, a rethink is required.

“This has been a hugely important day for us in terms of understanding what this division’s all about,” said Russell Martin after his side had almost 70% of possession but used the ball so ineffectively that they conceded five goals. “Sunderland fought much harder than us. I’m hurt, my ego’s dented.”

Southampton limbered up with the swagger of a side possessing 10 points from their opening four games. Within two hours, though, they had been ambushed by Sunderland’s counterattacking brilliance and all the talk of an immediate Premier League return seemed fanciful.

With Pierre Ekwah, the scorer of two fine goals, and Abdoullah Ba shining in midfield, Sunderland’s hitherto indifferent start to the season was rapidly forgotten.

Not that Tony Mowbray was getting remotely carried away. When, after 53 seconds, Jack Clarke – excellent throughout – connected with Trai Hume’s cross and headed Sunderland in front, their manager remained impassive. Rather than celebrate, Mowbray merely took a few sips from the water bottle placed at the edge of his technical area.

He picked that bottle up again in the seventh minute, reprising the same routine. Jobe Bellingham, Ba and Ekwah had just combined adroitly before Ekwah shot low, left-footed, beyond Gavin Bazunu from the edge of the area.

“Are you watching, Ross Stewart?” asked the crowd. On Friday, the Scotland striker had moved to Southampton from Sunderland for £10m with Mowbray acknowledging that Stewart had outgrown the wage structure.

Sunderland’s business model is to sign promising young players, some on loan, and task their manager with maximising such proteges’ value. It is a high-risk strategy but, thanks to much astute coaching, it swept a youthful side to last season’s playoff semi-finals.

Since then, plenty of experience has been sold with, most notably, Lynden Gooch relocated to Stoke and Danny Batth to Norwich, while signing further raw potential. The impressive 17-year-old Bellingham – Jude’s little brother – has arrived from Birmingham for around £2m while his fellow forward Mason Burstow watched on after completing a loan move from Chelsea on Friday.

Burstow is such a hot property that Milan considered signing the 20-year-old but now he can look forward to latching on to Ekwah’s passes.

Close to half-time Ekwah scored his second, slaloming through the defence before switching possession from his right to left foot and leaving Bazunu wrongfooted courtesy of a viciously swerving shot from outside the area.

Three minutes into the second half, Bazunu performed wonders to repel Bradley Dack’s header after Dennis Cirkin’s cross but the ball fell kindly for Dack to stab goal number four home from close range.

The fifth arrived five minutes into stoppage time when the 16-year-old substitute Chris Rigg became Sunderland’s youngest League goalscorer after meeting Jewison Bennette’s cross.

“I tell my players to go and express themselves, be positive and take people on,” said Mowbray. “I tell them not to worry about losing the ball. Football should be about individuals and flair. This is an important result for us.”