I don't think Brown was terrible today - he is physical and decent in the air, relatively composed and has a solid reading of the game (as evidenced by the one he got back to clear off the line).
But he is not a fullback. He's slow AF and not that fit, and is not going to be able to keep up with wingers running at him, especially when he's getting as little protection as he was getting from Sykes today. With Sam Long looking really shaky since he moved out to RB, and Seddon having been a bit of a defensive liability all season, we really don't have any good options at the fullback spots right now.
Honestly, if we kept everyone else from out current squad (not saying we'll be able to, but if we did) and just swapped out Hanson, Winnall and one of Holland/Whyte for two quality fullbacks and a solid defensive midfielder - I think we'd be in great shape for next year.
But we may well ship 60 goals this year - it's 56 already, and with two good teams to come, it seems probable.
As far as I can tell, the last team to concede 60+ in a year and still get promoted from the third tier was Peterborough in 2010/11 - and they had to score a frankly absurd 106 goals that season to do so.
We've simply got to sort it out at the back if we're serious about getting to the Championship.