How to test/mock Trailblazer operations that interact with external gems?

I'm loving all the object oriented beauty of Trailblazer!

I have an operation that interacts with a gem (called cpanel_deployer) to do something externally on the web. (It adds an addon domain to a cpanel.)

class Website::Deploy < Trailblazer::Operation
  attr_reader :website, :cpanel

  def process(params)
    real_cpanel_add_domain
    website.cpanel = cpanel
    website.save
  end

  private

  def setup!(params)
    @cpanel = Cpanel.find(params[:cpanel_id])
    @website = website.find(params[:website_id])
  end

  def real_cpanel_add_domain
    cp_domain = CpanelDeployer::Domain.new(website.domain)
    cp_panel = CpanelDeployer::Panel.new(cpanel.host, cpanel.username, cpanel.password)

    res = cp_panel.add_domain(cp_domain)

    raise StandardError unless res
  end

end

The cpanel_deloyer gem is already tested, so I don't need to retest it's functionality here. But in order to test the operation, I want to make sure CpanelDeployer::Panel#add_domain is called with correct args. So I'm thinking I should mock CpanelDeployer::Panel.

I believe it's bad practice to try to use any_instance_of. According to thoughtbot, it's usually considered code smell... They recommend using dependency injection. Is there a good way to use dependency injection within a trailblazer operation? Is there another best practice for this kind of situation?