Consulted my son (a physio) who watched the match with me. Like all medical professionals he was cautious and said it could be any number of injuries but when pressed on it, felt it was most likely initially a strain of the gastrocnemius muscle (calf) and then a rupture. Happens when athletes who run a lot and change direction to evade opponents are tired. Being on the older side can also be a factor. Probably he should have come off earlier but he's an important player and understandably wanted to stay out there. Recovery depends on degree of tear and treatment which can range from conservative to radical (surgery).
A damn shame. I had remarked to my son that this was the Valentine Holmes who played for Cronulla years before. Best attacking game I'd seen from him in Cowboys colours. The dynamic running, speed and step was back and would be a welcome addition to our outside backs which currently relied on slow behemoths like Suli and Rava playing like extra forwards.
All we can do is hope for the best and that he fully recovers. What we saw last night is what I want to see from him every week in Dragons colours.