The way I see it is this;
The SPL is a fairly rubbish league, but Celtic themselves are at least thereabouts to European standard, so their players can't be so easily discounted.
We're looking closely at Mangala who is a (recently) 23 yr old, 6'2, left-footed CB, who has played once for France as a full international and is (allegedly) valued at £38m.
We're also considering Van Dijk, who is a 22 yr old, 6'4, right-footed CB, who as yet - as far as I can tell - to play as a full international, but has been capped often at U21 level, and is valued somewhere around £10m.
Leaving aside the issue of whether we would prefer a left-sided CB to partner Vinny, the issue for me boils down to whether Mangala is £28m better than Van Dijk; both are supposedly 'commanding' and assured in possession, both have played plenty of international football, both are roughly the same age and both come from very lopsided leagues. Unless there is demonstrably a huge gap in class and ability, honestly I'd prefer the Dutchman.
With MDM very much a stop-gap, Rekik and the kids (i.e Denayer) unproven or not ready, Kompany becoming a tad injury-prone and Lescott on the way out, we could feasibly buy both, FFP notwithstanding.
Imagine a CB set of Kompany, Mangala, Van Dijk and Nastasić. Fun times.