Fielding's 3 month loan spell from Ewood Park ends in December and the boss did not want to be left short of goalkeeping cover.
The young German has not made a first team appearance at Old Trafford but was on the bench for a Carling Cup tie earlier in the season.
Gray told the club's official website: "Under the terms of the loan ruling, Frank Fielding will have to go back to Blackburn after the Scunthorpe game.
"His 93 days will be up then and he will go back to his parent club. With that in mind, we would have been left with only one professional keeper in Chris Dunn and a youth scholar in Paul Walker between then and when the January transfer window opens, so we have taken Ron-Robert in before the loan window closed to help us out in that period.
"He is another young player with a fantastic pedigree and we are looking forward to him joining us."