Former NFL head team doctor and practising orthopaedic surgeon David J. Chao says Hector Bellerin appears to have torn his left knee quad tendon, based on the video of the injury circulating on social media.

Bellerin had only just returned from a calf injury to make his first start of the new year against Chelsea over the weekend. However, after a good performance as Arsenal raced to a two-goal lead, the right-back suffered a new blow that forced him off the pitch on a stretcher.
The immediate concern was that the defender might have torn his ACL, as Rob Holding did recently. That injury would require an extended period on the sidelines, possibly even up to a year out. Chao provided a different diagnosis based on his own viewing of the footage:
By video, #HectorBellerin @Arsenal tears his left knee quad tendon. Doesn't appear to be ACL, but still surgery and at least as severe. https://t.co/LFcHl5F7oA
— David J. Chao, MD (@ProFootballDoc) January 19, 2019
*Bellerin
— David J. Chao, MD (@ProFootballDoc) January 19, 2019
Chao isn’t the only one with theories though, as another popular assessment on social media right now is that it could be a Lateral Collateral Ligament (LCL) injury. That would be the quickest recovery time of the three alternatives.
https://twitter.com/AFCAMDEN/status/1086730408258125824
Once again, these are only assessments made based on one replay from one angle on Twitter. It’s worth waiting for Arsenal’s official tests and scans before jumping to conclusions.