In the headliner bout at UFC 213, Robert Whittaker suffered what appears to be an injury to his left knee in the first round, thanks to a couple of well-placed push (or oblique) kicks by Yoel Romero. As of this writing, there is no word as to the extent of the damage, but Whittaker was apparently heard to tell his corner between rounds 1 and 2 "my leg is trashed." (I say 'apparently' because I was not able to watch the PPV portion of this card, and relied on media reports for the specifics of this bout. That, and I'm too cheap to buy the PPV after the fact on FightPass.) In an interview with an Australian news outlet posted July 11, Whittaker exclaimed "One more kick on that knee, I was gone." He is now facing a lengthy medical suspension, pending clearance by an orthopedic physician.
On review of highlight video of this fight, it seems fairly clear what caused the damage. Romero hit Whittaker with at least one push kick placed right on the knee. Below is a screen capture of the kick.
In the image, you can see the impact on Whittaker's left knee coming from an anterior and lateral direction. This forces his knee into an inward (valgus) and posterior (hyperextension) direction. The potential for damage to many structures of the knee is significant. The anterior and posterior cruciate ligaments within the joint capsule could both be at risk, as well as the medial collateral ligament that lies external to the capsule, on the inside of the knee.
Furthermore, Whittaker could have sustained damage to the posteromedial corner (PMC) of the knee. The location of this 'corner' is at the back and inside part of the knee, as shown in the picture below.
We don't hear much about the PMC in injury discussions, but there are important structures here that, when damaged, can have a serious impact on the stability of the knee. These structures include (among others) the semimembranosus tendon insertion (which is one of the hamstring muscles), the posteromedial capsule of the knee joint, and the posterior horn of the medial meniscus. Damage to the PMC usually occurs in conjunction with damage to other structures of the knee, including the ACL, MCL, and/or PCL. From a functional standpoint, PMC injury can contribute to instability of the knee, particularly anteromedial rotary instability, or AMRI. This occurs due to excessive external (outward) rotation and anterior (forward) translation of the tibia in relation to the femur (see image below).
In summary, the tremendous force with which Yoel Romero kicked Whittaker's left knee has the potential to inflict serious damage, especially on a knee that had reportedly already been injured in training leading up to this fight. My goal here has been not to give a definitive diagnosis of Whittaker's knee injury, but only to analyze the injury possibilities and, hopefully, provide some education to the reader.
Lundquist RB, Matcuk GR et al. Posteromedial corner of the knee: the neglected corner. Radiographics. 2015;35(4): 1123-1137.