We've been incredibly fortunate to get through last season with essentially just 6 defenders, It's about 3 light. Some teams had more than 6 defenders injured at once at times over the season.
Even if you throw lewis and gomez in as defenders which they're not, it's still light, most other squads have 8-10 defenders in their squad. Stones is unreliable with injuries, Walker is at a stage now where his game time needs to be managed, he just can't be playing every game anymore. There's not just plenty of space, but there's a strong need for defensive reinforcements.
I don't really think frimpong is the answer for us, just doesn't fit, he's more of a wing back. we've got Cuoto on the books still that we don't appear to be keen on integrating into the side despite interest from Madrid and we passed on bring Porro back last year, both of whom I'd categorise as similar in style hlto Frimpong, we definitely need reinforcements in that area though.