The rumour mill seems rife with defensive recruitment potential. And my goodness is that the correct priority!
If we summarise 2018/19 defensive performances, the extent of the hole in the Celtic rear-guard becomes stark.
The chart below maps DASR% and Possession Win %.
These are both aggregated metrics. Defensive Action Success Rate % considers all defensive actions (not passes) and denotes the extent to which the player was successful in winning them or not.
Possession Win % considers the extent to which defensive actions (not passes) resulted in Celtic maintaining possession.
I like these because they match the eye test. That is, what is observed is supported by the data.

The players in Red are no longer here.
Benkovic ended the season in Van Dijk territory with his DASR% of 81%. He was that good.
Boyata had a better Possession Win % and slightly lower DASR%.
But Boyata and Benkovic, by the performance data, were the number one and two centre backs. Both gone.
The good news is Ajer and Simunovic, both young players, are not far behind. Then the picking are slim. Hendry is just not near that standard.
Of the full backs, the experience of Lustig (best full back at defending) and Izaguirre are now gone.
Tierney’s numbers are always lower due to the amount of attacking he does. I don’t record where challenges occur but because Tierney loses challenges in the opposition half through attacking, this makes his defensive actions stats lower. In short, by these measures, he is an outlier.
As we can see Toljan is not a great loss and Ralston (less than 5 games worth of data) is just behind Lustig.
It is patently obvious Celtic need defenders, but hopefully this provides some empirical evidence to show you the scale of the squad management challenge.