With everyone fit from last season, I'd have said our best central defensive partnership was probably Seaborne and Frans. We still have both of them. If you take Lindsay as being good enough for 3rd choice (which he is, IMO) then I don't think we're particularly weaker in central defence. We've lost Taylor, but that was always likely to happen and we're yet to see who will replace him with 2 weeks or so left until the start of the season, so it's still very possible that we could have a central attack that is on par with what we had last season.
If you add in that we've improved our keeper, Gary Miller looks to be signing, who while not as good as SOD was is still very much competent at this level, and we've also brought in Amoo who looks to be a good player and Welsh coming back (hopefully) strengthens our midfield options then I don't really think we're particularly weaker than at the end of last season at all. Of our best performing players from last season IMO, only SOD has left and it seems we're getting as good a replacement as you could realistically hope for.
Still 2 weeks to go but if we can get a couple of players in then we should be set for a decent season.