I don't know much about what is available in Play, but from the REST API documentation, here is a workaround if your library doesn't support PATCH:
If you use an HTTP library that doesn't allow overriding or setting an arbitrary HTTP method name, you can send a POST request and provide an override to the HTTP method via the query string parameter _HttpMethod.
For example, to update an Account, this will work with an actual POST request:
As for the reasoning behind using PATCH, it is because PATCH is for partial updates to a resource. That is, you only have to send the fields you are updating. If you were required to send all the fields for a record in updates, PUT would probably be a better choice. POST is generally only for new inserts. Here is an explanation with examples: http://jasonsirota.com/rest-partial-updates-use-post-put-or-patch
To learn in-depth about Salesforce, sign up for an industry based Salesforce Certification.