I'm sure AN will have looked into the possibility of signing a centre back this summer but if we want quality acquisitions that will improve our team, finding a centre back that fulfils that statement is tricky to find, especially given our relatively limited budget. Brady improves us. Mulumbu improves us. Dorrans improves us.
I believe van Dijk would improve our starting eleven, he's a talented lad, but I'm not surprised clubs like Everton and Southampton are in for him and even if they weren't, the £10m asking price will likely be out of our reach. AN isn't going to sign a centre half who he doesn't feel offers us something better. Neil Adams did that last summer and wasted fees and wages on Cuellar and the hideously bad Miquel.
I personally would be spending the rest of our budget on a centre half, although as someone who was very critical of Martin playing centre back last season - mainly because he was dire when playing there initially - I actually have less concerns about him next season than I did a year ago. Not only do I think he has improved in that role - mainly because he has Bassong alongside him I suspect - I think he is more suited to the Premier League as he does read the game fairly well.
Like I say, in an ideal world we would bring in a top centre back and partner Bassong with him - but it might not be possible.