“I’m knackered,” Robin Gosens told Sky. “But very, very happy.” What started out looking like the most forgiving, gentle induction into life together for him and Union Berlin didn’t turn out quite like that. It was something far more satisfying instead.

Gosens had waited a long time for this, his full Bundesliga debut, just like Union have spent a long while working themselves into this position. No longer the underdogs but the ogres, they ruthlessly crushed the hopes of Darmstadt in the promoted club’s first game at their Böllenfalltor home since arriving back in the big time. Union did what they have often done in the past year or so. They found themselves in a tough spot and decisively chopped their way out of the woods, in a manner not dissimilar to vintage Bayern Munich.

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Signing Gosens – likewise Kevin Volland – represents the next stage in their development, bringing in players of established pedigree. The club’s sporting director, Oliver Ruhnert, also confirmed that Union are exploring the possible transfer of Leonardo Bonucci from Juventus. “He’s not here yet,” a cautiously optimistic Ruhnert told ZDF’s Das Akteulle Sportstudio on Saturday.

The hugely experienced Italy centre-back would join a list of illustrious arrivals that would have seemed impossible a few years ago, but with their market smarts, Union have a nose for an opportunity. They are not going wild spending money they don’t have. That Juve are willing to release Bonucci from the last year of his contract for free was, Ruhnert admits, a big part of the attraction.

Yet Gosens stands out among this summer’s new recruits, and will continue so to do. He’s different. Before last week’s cameo in the 4-1 win over Mainz, his last competitive appearance had been in the Champions League final for Inter against Manchester City – in which he nearly equalised with virtually the final touch of the match. Even if Ruhnert tried talking down the reported €15m fee on ZDF – it was nearer €13m with add-ons, he took care to say – it felt like a signalling of intent.

Not that the Germany international has always been at the top table – far from it. Playing for regional sides as a teenager, the closest he got to a big break was a curtailed trial at Borussia Dortmund – “It was a fiasco. I couldn’t keep up at all,” he told T-Online back in 2018 – before he belatedly found a way into the professional game, over the border in the Netherlands but closer to his Emmerich home, at the Vitesse Arnhem academy. But after finally signing a professional deal at 19, he didn’t make a single first-team appearance for Vitesse and moved to Heracles on a free transfer in 2015.

After joining Atalanta in 2017, he established himself as one of the outstanding wing-backs in Serie A as part of Gian Piero Gasperini’s buccaneering team. He even made his full international debut during his time in Bergamo. Yet Gosens’ path has never stayed easy for long. His move to Inter on an initial loan in January 2022 should have been a coronation of sorts, but arriving with an injury, he struggled to make an impact. Inter probably would have sent him back to Atalanta had the obligatory purchase criteria not swiftly been met.

So if Saturday was all about Gosens fulfilling a childhood dream, making a first Bundesliga start at the age of 29, it also represented a recurrent theme in his career: the top-level player who always has to prove himself. A typical Union success story, in his own way. Gosens quickly had lift-off, scoring in the fourth minute to put Union in front, but for player and team it turned out to not be as simple as that lightning start suggested. Just past the 20-minute mark another new arrival, the midfielder Brenden Aaronson, was sent off for a second bookable offence and, with the visitors still recalibrating, Marvin Mehlem finished off a delightful move to equalise for Darmstadt just three minutes later.

Gosens, though, knew this was his day. Having opened the scoring with a typical sweep of his trusty left foot, he towered to place in a near-post header from a Josip Juranovic free-kick that put Union back in front. Kevin Behrens, fresh from last week’s hat-trick against Mainz, then scored from close range – with his head again, obviously – before half-time to bring a bit of order to proceedings, and Danilho Doekhi’s brave header midway through the second half wrapped things up.

“Personally, I can talk about it being a perfect day,” Gosens said, with some satisfaction. “[In the] starting XI in the Bundesliga, a 4-1 win with a man down, two goals for me … I don’t think much more is possible.” It was a big moment for Union, too – they now have two wins from two.

A pair of 4-1 wins make it look easy on paper but really, they have been anything but. On a weekend in which Dortmund suffered their first stumble of the season with a draw at Bochum, the value of fortitude in adversity was never clearer. And to top it all off for Gosens, the national team coach, Hansi Flick, was in the stands, watching on as he gave his recital.

“I had no reason to be nervous,” he said with a smile, “because I didn’t know.” Like Union themselves, after everything he went through to get here, Gosens is laser-focused on making the most of unexpected opportunities this season.

Talking points

• Harry Kane made his home Bundesliga bow for Bayern, having told Bild am Sonntag he “wants to be a leader” for his new club. The striker led from the front with a double in a 3-1 win over determined Augsburg, whose football was memorably described as “disgusting” and “not great for the spectators” by Leon Goretzka. Bayern still have work to do in the transfer market and could use an extra midfielder, though Manuel Neuer’s participation in full training on Monday was a huge boost.

• As for the challengers, just as they had been during last season’s title run-in, Dortmund were stymied in the mini-derby at Bochum. Again, they went a goal down early on – to a rocket of a strike by Kevin Stöger – before Donyell Malen saved a point. “The performance was just not enough,” chided Edin Terzic. Leipzig, and the already inevitable Xavi Simons, came from a goal down to thrash Stuttgart 5-1 on Friday.

• Could Leverkusen also be in the title mix? It seemed fanciful after the summer sale of Moussa Diaby, but the authoritative way in which they dismissed Borussia Mönchengladbach in Saturday evening’s Top-Spiel raised the question. Victor Boniface shone with a double, scoring with two of the 11 shots he had against feeble opposition and Xabi Alonso’s team also completed their transfer business by getting Nathan Tella, their de facto Diaby replacement, on an initial €20m deal from Southampton.

• It was home sweet home for Maximilian Philipp on his return to Freiburg after six years away. On as a substitute, he popped up in the sixth minute of stoppage time with a sumptuous finish for the winner against Werder Bremen – the team for which he played last season.

Friday: RB Leipzig 5-1 Stuttgart. Sat: Bochum 1-1 Dortmund; Cologne 1-2 Wolfsburg; Darmstadt 1-4 Union Berlin; Freiburg 1-0 Werder; Gladbach 0-3 Leverkusen; Heidenheim 2-3 Hoffenheim. Sun: Bayern 3-1 Augsburg; Mainz 1-1 Eintracht.