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.
Big blow: Danny Welbeck is unlikely to return until mid-March
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.'
Welbeck picked up the injury in the closing stages of the 1-0 win at Aston Villa on January 5.
The injury was initially diagnosed as a hamstring problem - but the operation has given a different picture.
It is a blow for the 20-year-old, who has been in impressive form since scoring his first goal for the Wearsiders in the 3-0 win at Chelsea in November.
That breakthrough at Stamford Bridge was the first of six goals in nine games which leaves him second in the club's Premier League goalscoring charts behind Gyan, following this week's departure of Bent.
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