I'm undecided.. On one hand Stones is weak, clever but physically he struggles. Mori, who I rate highly, needs to work on positioning sometimes I feel and Jagielka, well I feel like Jags is in that period now where he could fall away with quality (some argue he already has). If we could go and sign a top defender, old fashioned then I would.
Even with Koeman, a manager a bit more defensively sound I still worry about the quality we have defensively. Especially when none of our back 5/6 players have been on form in the past two years. (Mori aside considering he's only a year old into his Everton career).