It is now more like a hernia rather than a milder tear. Which means that Bendtner needs surgical intervention and is likely to be out for 6 weeks or more.
Thomas Vermaelen and William Gallas have been very good so far on set pieces but Bendtner would have given an aerial threat to get around Chelsea's physical defense (the Blues visit the Emirates on Nov 29th) and win back possession. His touch has also gotten better. So this is is a significant blow.