'This Is Us' Star Milo Ventimiglia Joins Mysterious Cast

1:28 PM PT -- Milo told Jimmy Fallon he had surgery to repair torn tendons 'The Tonight Show.'

Milo Ventimiglia dies on his show, "This Is Us," although we don't know how ... we also don't know how he broke his arm.

We got the Pearsons' flashback father Tuesday night in NYC, where he was swarmed by fans wanting his John Hancock, but that presented a special problem, because of some busted bones.

Milo's mum on the subject, but he made one thing clear ... he loves those who clearly love him.