Knox is currently trialling with United.
“Matthew left on Sunday. That's why he didn't play in the game on Saturday against Stenhousemuir," said Hopkin.
“He will be down there until Friday. Man United asked him to go back down again which is obviously great for him.
“He'll be training for the whole week and he's got a bounce game with the first team today, which is terrific.
“It's a great honour for him to be training with the team and be invited down again. He must have done something to impress him last time. He's handled everything great here and he's a real talent.
“We'll find out towards the end of the week exactly what's happening, but it's all positive for him."