The 33-year-old joined the Clarets in September as a free agent after being released by Preston at the end of last season.
Lindegaard was drafted in to provide cover after first choice Tom Heaton suffered a dislocated shoulder, which has ruled him out "for months".
Nick Pope has been trusted to deputise in Heaton's absence and Lindegaard is waiting in the wings if called on by Sean Dyche.
The former Manchester United stopper has kept two clean sheets while featuring for Burnley's under-23 side and Duff has been impressed by his exploits.
Duff said: "He showed his quality in the last five minutes when the ball was coming into the box. His kicking was excellent.
"Considering he's been out for a while, I think it was his second game in (11 months), but you could see his quality at the end, coming for crosses and diffusing the situation when we were under the cosh. He showed what he's all about."