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Welbeck, who is on loan from Manchester United for the season, underwent an operation on Wednesday to repair a torn knee cartilage a day after former team-mate Darren Bent had sealed his big-money switch to Aston Villa.
That leaves Bruce with only one fit senior striker - record signing Asamoah Gyan - for Saturday's Barclays Premier League trip to Blackpool.
Bruce said: "Danny was operated on yesterday and it was confirmed he had a torn cartilage and he will be [out for] six to eight weeks."