The on-loan Manchester United youngster was dropped to the bench following last Wednesday's home defeat to Bolton but sparked a hitherto lacklustre Rovers when he replaced Morten Gamst Pedersen after 55 minutes at Turf Moor.
"Josh, when he came on, made a very good impact," said Berg.
"He gave us a bit more attacking options, a bit more offensive play and (is) also someone who can take on and go past players.
"I think he had an impact in his first home game (against Millwall) as well when he came on, he did really well getting into great positions.
"He's strong, he's physical, he's got good technique and against Burnley his passing was good as well.
"It's good for him to make that impact because he really changed the game a little bit for us."