Must We Directly Address a Demon to Make It Leave?

Not necessarily, though there are more cases in the New Testament where a command was given than cases where we know that no command was given.  There are other cases where the details are not specified or the Bible simply says that the demonized person was “healed.”

In Luke 13:11-16, Jesus never directly addressed the spirit of infirmity that was in the woman.  He only said, “Woman, thou art loosed from thine infirmity.”  In order for His word to come to pass, the demon had to leave, and it did.

In Acts 19:11-12, the anointing, transmitted by cloth items, drove demons out with any record that anyone spoke to them directly.

In Matthew 15:22-28 and Mark 7:25-30, Jesus simply said, “Be it unto thee even as thou wilt” and “the devil is gone out of thy daughter” and the demon, which was far away from where Jesus was at time, left.

Here is a summary of which verses fall into which category:

Spoke to the Demon  
Matthew 8:16-17                         
Matthew 8:28-32                                                
Matthew 17:14-21                                    
Mark 1:23-26                                              
Mark 1:32-34                                                                            
Mark 3:10-12                                                                        
Mark 5:2-20                                                                          
Mark 9:17-29                                                                         
Luke 4:33-36
Luke 4:40-41
Luke 8:27-39
Luke 9:38-42
Luke 11:14
Acts 16:16-18

Did Not Speak to the Demon
Matthew 15:22-28
Mark 7:25-30
Luke 13:11-16
Acts 19:11-12 

Unspecified
Matthew 9:32-33
Mark 1:39
Mark 6:12-13
Mark 9:38-39
Luke 8:2
Luke 9:49-50
Acts 8:5-8

“Healed”
Matthew 4:23-24
Matthew 12:22
Luke 7:20-23
Acts 5:12-16