“When it all comes together, Dusan Tadic will be a fantastic reinforcement, a top professional,” Ten Hag said last June when asked about reports of an imminent €11.4 million (£9.8m/$12.8m) deal.
“That makes him an example for his fellow players, both on and off the field. We have many young players, many big talents that the top teams in Europe are after. They can fit together perfectly with such a player to help Ajax win trophies.
“Tadic has great individual qualities. He always knows how to choose the right position, sees the final pass and can play football well. It is a pleasure to watch. He has a phenomenal kick and thereby gets returns in his game.”
There was little doubt he would be a star in Amsterdam. After all, Tadic had spent four years as one of the Eredivisie’s best players in spells at Groningen and Twente.
A master at supplying team-mates with great chances, he made 19 assists in his first season in the Dutch top flight. That creativity has stayed with him, but he found his scoring touch at Twente, where he scored 32 goals to go with his 31 assists in 85 appearances.
His time at Southampton may have been hindered by inconsistency, but he was one of their most productive and creative players in each of his four seasons at St Mary’s.
Expectations were high upon his return to Netherlands, but not even Ten Hag could have predicted the winger would prove such an inspiration to a free-scoring team gunning for a domestic double and disrupting Europe’s elite in the Champions League.
He has been the star of the competition this season amid Ajax’s surprise run to the quarter-finals. Only Lionel Messi (11) has been directly involved in more Champions League goals than Tadic (nine) and only three players – Toni Kroos, Memphis Depay and Raheem Sterling – have created more chances than the Serb (22).
But it was his role as the mastermind behind one of Ajax’s all-time great European nights that best illustrates the importance of the refined, now fully developed Tadic.
Unlucky to lose 2-1 at home to three-time reigning champions Real Madrid in the first leg of their last-16 tie, Ajax’s campaign looked all but over heading into the return at Santiago Bernabeu.
Just seven minutes in, though, Tadic nipped ahead of Nacho to carry a loose ball into the box and squared to Hakim Ziyech for a fine finish to bring them back into it.
Ten minutes later, the 30-year-old stunned the crowd when he evaded Casemiro with an awesome pirouette, strolling into the space outside the box and sending David Neres through to make it 2-0.
It all got surreal after the hour mark when he rounded off an excellent team move with a perfect shot into the top corner.
The 4-1 victory propelled Ajax into the quarter-finals and Tadic into prominence, earning him a rare 10/10 rating from famously stingy French sports daily L’Equipe .
“We have our own Cristiano Ronaldo: Dusan Tadic!” said captain Matthijs de Ligt , and they will need him to maintain that calibre of performance when they take on the real thing and his Juventus team in the next round.
While Ronaldo has been working on his recovery for the European showdown, Tadic has maintained his excellent form for Ajax.
He was sublime against Fortuna Sittard, pivotal against PEC Zwolle, scored against PSV and set up the team’s 99th and 100th league goals in Saturday’s 4-1 demolition of Willem II.
With 30 goals and 17 assists in 46 appearances in all competitions, Tadic has filled the leading role on the field that Ten Hag expected. There’s more to Ajax than Tadic, however, and vice versa.
Leading the charge alongside Ziyech, Neres and sometimes Klaas-Jan Huntelaar, Tadic completes a fast, unpredictable and clinical attack, backed by Donny van de Beek and Frenkie de Jong in midfield. With full-backs Nicolas Tagliafico and Noussair Mazraoui charging forward too, Ajax can threaten from all sides.
The freedom they play with and the wealth of passing options available suits Tadic perfectly.
"I'm really happy to be here, because I'm playing with good players and the philosophy of the club is beautiful," he said in September . "That is the most important thing. If you find the click, you can play lovely football and then it doesn't matter what they say about you.”
It’s not just the football in the Dutch capital that suits him, he is happier living in Netherlands than he was in London.
"My family would pick Amsterdam over England every day," he said. "It's a beautiful city. There's nothing I don't like about this place.
"Netherlands is a way better place to live in than England. They only have London, but here it's different. Everything is clean and there are nice roads, streets and bars here. It's something special.”
It is the perfect environment to spend his peak years and, as the fourth oldest in the squad, he makes for an excellent role model for precocious stars like Barcelona-bound De Jong or 19-year-old sensation De Ligt.
"I very rarely get impressed with the work ethic of a colleague, but Dusan has been at a higher level in this regard than any other player I've seen in my career," former team-mate Tim Sparv said.
"In Groningen, there was a small gym beside the dressing room and I can still remember how Dusan worked there individually before and after each training session. This was not a club requirement, it was his own motivation."
A relentless gym rat, proficient scorer and excellent creator setting an example for those around him? He really is Ajax's Ronaldo.