I believe it is generally best to be upfront. Your attorney can send a letter at least advising the probation officer that you have entered rehab and asking for her to contact him if there is any problem. You don't want the probation officer not to be aware of where you are and miss a probation obligation without giving her notice.
Most probation officers (and judges) would be thrilled if their defendants voluntarily entered a rehab program if they have a problem that can be helped by rehab program. Therefore, if you think your probation officer may react differently, then you need to discuss your plans immediately with your previous attorney or retain new experienced criminal defense counsel to help you deal with your specific situation. It will probably be more expensive and more exposure to your liberty, if you wait and address the issue, after the fact, at a probation violation hearing.
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You absolutely need to make the court and your probation officer aware that you are entering treatment. Missing a check-in can be an automatic violation, depending on the severity of your case and the extent of your probation. It is better to give a heads up than to try to explain later. That being said, judges can be more lenient when dealing with a defendant willing to seek treatment. As my colleagues have suggested, consult with your attorney or hire a new one to help you with this. You don't want to risk your probation on a missed check-in, even if it is because you are in treatment.
You need to contact the probation officer and let her/him know that you are entering treatment. I cant imagine the probation department denying you the ability to receive treatment. Not contacting them could lead to your probation being revoked and you being sentenced to the maximum penalty for the conviction.
This is general information and not legal advise. This communication does not create a lawyer-client relationship.