The injury occurred after a 50-50 challenge with Vlasic on 22 minutes of the Premier League match, which left the Burnley man in agony and signalling to the bench.
He was treated for 10 minutes on the pitch by the medical staff before being stretchered off to a standing ovation by both sets of fans at the London Stadium.
Vlasic was almost in tears after witnessing the injury, while other players were also visibly upset by the incident.
Burnley caretaker manager Mike Jackson confirmed to Sky Sports in his post-match interview following the 1-1 draw that Westwood had been taken to hospital.
“He has gone to hospital and they are having a look at it at this moment in time,” he said. “Obviously we know it doesn’t look too good, but fingers crossed it’s not as bad [as it seemed].
“The incident with Ash affected the group… because he’s their mate. We’re just hoping he’s okay. Given everything that has gone on this week and what has happened in the game today, it’s a good point for us, in the end.”
Speaking after the match, West Ham midfielder Tomas Soucek said: “It was a horrible injury and I just hope Westwood feels very good, very soon. I just wish him all the best.”
Speaking in his post-match press conference, Jackson revealed West Ham captain Declan Rice came over to the Burnley bench to alert them to the seriousness of Westwood’s injury.
The stand-in Clarets boss said: “Declan Rice came over and said straight away that it’s not good.
“Straight away you can see from the reaction of the players and the lad himself, who I’ve seen put his hand up, and he doesn’t go down.
“It’s not good, but they’re a good group and they came together, but our thoughts are with Ash at this moment in time.”
Asked whether Westwood had suffered a serious ankle injury, Jackson responded: “Honestly, I couldn’t tell you.
