If you want to sell your book online or in bookstores, place it with distributors and wholesalers, or put it in libraries, then you'll need to have a separate ISBN for print and digital versions.
To adequately track your sales and distribute properly to online retailers, you will need one ISBN number for printed books and a different ISBN for your eBook. (Similarly, the hard cover version of a title and the soft cover version of a title would each need to have different unique ISBNs.)