The Reds halted title-chasing Sheffield Wednesday's 23-game unbeaten league run in a thrilling 4-2 win at Oakwell live on Sky Sports on Tuesday night and catapulted themselves into top-two contention. Barnsley, rebuilding under Duff, who they appointed in June after relegation, extended their own unbeaten run to 12 matches, which includes 10 wins.
The veteran goalkeeper played in a behind-closed-doors friendly on Thursday.
The Scots begin their campaign at home to Cyprus on Saturday knowing they are already assured of a play-off for next year's finals in Germany by virtue of their Nations League performance last year. "This is the toughest group in the whole Euro qualifiers I think but we're looking forward to the challenge," said the Liverpool left-back.