I'll stick to central defenders as that was your original argument....
Carvalho who I always had down as a reluctant taxi driver more than a footballer was a great signing for Chelsea, no denying that, but it's not as though you can give Mourinho any credit for signing him as he'd been his player at Porto for years previously and nobody else would've been prepared to spend the near £20m Chelsea paid for a 28/29 year old who had never played outside Portugal before.
There's still time for Zouma yet, but Chelsea unlike a few clubs, United included, could see past the fact the guy had a charge for battering his 15 year old girlfriend hanging over him when he was available and walked away from any possible deal, and rightly so, but we'll hear a lot more of him over the next few years of that I have no doubt.
Ok, let's stick to CB's. He recalled Jorge Costa from loan at Porto - a key member in the CL campaign, EL campaign as well as the two league titles.
Formed a great pair along with Carvalho at Porto, then he took him at Chelsea(regardless of you want to give him credit or not that was a masterstroke considering how good the Chelsea defence was at the time).
At Chelsea he made Terry captain and along with Carvalho formed the CB pair with years to come.
Up until he was sacked he only brought Alex as a CB (Boulahrouz was used as a RB) and he didn't even buy him as he returned from loan.
At Inter he only bought Lucio - and yes he was great that season.
At Real he brought in Carvalho for 8m EUR - by no means failure - they had the best defence in CL and bettered their defensive record in the league as well compared to the previous year under Pellegrini.
Next year he bought Varane for 10m EUR - another coup.
In his last Real year he promoted Nacho.
In his second stint at Chelsea he bought Zouma in the winter and Djilobodji next season for 2.7m pounds, later sold him for 8m pounds to Sunderland a year later, tripling his value.
At United he bought Lindelof and Bailly.
So please do tell us how his CB record isn't close to perfect?