Purely because you asked so I did a quick search:
John Collins, the assistant manager his time at Celtic:
Virgil Van Dijk is as good as Europe's best defenders (From HeraldScotland)
"I believe there is currently no bettercentre-half in Europe than Virgil Van Dijk. I watch Champions League football, I watch Premier League football every week and Virgil is as good as anybody who is playing football in Europe."
Current Hibs manager Neal Lennon, the man who brought him to Britain:
Virgil van Dijk: Southampton star tipped for Real Madrid or Barcelona moves | Football | Sport | Express.co.uk
When asked if Van Dijk can become the best defender in the world, Lennon told Sky Sports: "Yes."I think he could play for Barcelona, I think he could play for Real Madrid - it wouldn't surprise me if he took his career on to those pathways
."I couldn't believe we got him for €2.2m (£1.9m). He was outstanding. I couldn't believe that none of the big clubs took a bite at him before Southampton.
"I believe he could play at any club side in Europe now - he's that good. He's only 25, he's got 10 years in him.
Along with a multitude of quotes like this from different people within the game saying he's got everything to become World class, many suggesting he could play for any big club and then he would be:

Southampton FC
✔@SouthamptonFC
Puel: "I think @VirgilvDijk has all of the qualities to become one of the best defenders in the world." #saintsfc
12:12 PM - 4 Nov 2016
Does the above mean much of anything? No more or less than your opinion or mine. Again, opinions of players are subjective.
But that's three guys who've coached him first hand and I think it's fair to say they know far more than you or I or any other layman here.
So as you posed the question there's some answers for you.