I think you may have the following options:
Web Service for Remote Portlets 2.0 (WSRP)
Quoting from WSRP specification:
The Web Services for Remote Portlets specification defines a web
service interface for accessing and interacting with interactive
presentation-oriented web services.
Basically you need a running instance of Liferay exposing your portlets as presentation-oriented web service to Drupal that will consume them and send back to the client.
The good news is that Drupal seems to support it. You need to enable Drupal as a consumer of WSRP, see that Drupal page, and Liferay as a producer, see here.
Using IFrame
Similar to first solution but with less integration and more work to do about security because the client will contact directly the Liferay server, so you'll need to expose it as a public server (if not in a Intranet scenario) and you'll probably need a Single Sign On solution for authentication.
I suggest to take a look at that document about Liferay Application Integration Strategies because recap very well several integration strategies with pros and cons (including the two I cited).