If you run to your manager and say that you cannot handle an assignment without even trying, you are going to look weak in your manager's eyes. In addition, you are going to look weak in your own eyes.
Your manager has given you a vote of confidence, and you should not veto his decision without giving your additional role a try. In order to increase the likelihood of turning a try into a triumph, you should start by meeting with the manager of this department several times to discuss the department's operations, objectives, and procedures. If there are any manuals, be sure to review them. You should also meet with some of the key employees in the department to discuss what they are doing, and you may even want to contact the former manager to pick his or her brain. If there is formal training available, you should attend some sessions.
With this foundation in place, you will be in a much better position to determine if these additional responsibilities should be part of your position.
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