Selena Gomez Stalker -- Locked Up ... But Just For a Little While

The stalker who has shown up repeatedly at Selena Gomez' house and actually entered her property has just been ordered to get treatment in a mental health facility ... but the bad news for Selena ... he could be out in as early as 30 days.

Che Cruz pled no contest Friday to felony stalking. He was ordered to a mental health facility where he will be on lockdown for 30 days. After that, he must complete a total of 180 days at the facility, but he's not required to stay there.

The problem, of course ... Cruz is clearly unstable and everything hinges long-term on him finishing the program. And, he's proven already ... once out of custody, he makes a beeline for Selena.

The judge also issued an unusual, 10-year protective order, requiring Cruz to stay 200 yards from Selena. Again ... he's already violated a protective order.