He's got 37 goals and 17 assists in 91 games in the Championship, so he's clearly proven at this level. It may be that McCann believes he can attract better for Dundee in the Premiership, so is happy to let him leave and use his wage on someone else. You'd need to ask Dee fans how he's actually performed for them, but it seems like he's been tarred with the same brush as Keatings i.e. capable at this level, but not trusted to make the step up.