The Rovers boss made his first signing with the arrival of the Norwegian international striker on a six-week loan deal to January 2nd.
"We are very happy to bring Joshua in," said Berg. "He will add pace, strength and more attacking options to the team.
"We are going into a period of eight games against very very good teams, so we are happy to get him here and happy with United for letting him come to us.
"He's been there for five years, he's been on loan at other places and now he is coming to us and we are delighted with that.
"I also think it is good for Joshua as well to be able to play more first team football and to continue to develop as a player."
See Joshua King and the rest of the Rovers squad this coming Saturday when Millwall are the visitors to Ewood Park.
Tickets are on sale right now from £15 for Adults, £10 Seniors and £5 Juniors. Book online today at www.onerovers.co.uk
and collect your tickets from the Blackburn End collection point on the day of the game.