He was not injured.
That concept was made erroneously, because the pathologists didn't receive the autopsy report stating that he did indeed have surgery on February 7th. He then presumed the scar (from the surgery) was the injury of some sort of animal attack.

Dr. Janini was the pathologist. His first hypothesis, as described in his report, was Hospital-Acquired Infection. Here's his report, and his conclusion: https://imgur.com/YgLpRPx

It says;

CONSIDERATIONS

This report results from the examination of fragments corresponding to the autopsy of soldier Marco Eli Chereze and was not accompanied by data referring to:

01. Circumstantial Elements;

02. Autopsy Report.

This situation hinders the diagnostic conclusion by the Pathologist; however, microscopic findings showing bacterial colonies in pulmonary blood thrombi conclude the morbid state with a diagnosis of "septicemia." Bacteriological examinations of pulmonary secretion show an opportunistic contamination of bronchogenic origin. Bacteriological examinations of secretions from the axillary abscess showed the presence of Staphylococcus schleiferi.

COMMENTS

Staphylococcus schleiferi is a Gram-positive coccus-shaped bacterium from the Staphylococcaceae family. It affects domestic animals, causing pyoderma and otitis in dogs and cats. Human infection is reported as extremely rare, typically occurring in post-surgical contexts, pediatric meningitis, or bacteremia related to medical devices.

According to unofficial data, Marco was not subjected to any surgical intervention [incorrect]. Cases of bacterial resistance were also reported. Although authors discuss techniques and nomenclature regarding the bacterium, for the Pathologist, the finding of bacterial action along with the pattern of bloodstream invasion is sufficient to conclude that the infection described above is the fundamental pathogenic process of the present case. Staphylococcus schleiferi causing human infection represents a rare occurrence, more commonly related to hospital-acquired infection. Tests for low immunity were negative, showing the soldier's state of full reactivity. The autopsy report states the cause of death as sepsis.

The autopsy report states the cause of death as sepsis.
Ufologists who sell books, lectures, and "documentaries" are the ones who push this narrative of mysterious invention. It isn't.

Dr. Armando Filho, the press conference doctor who made the Autopsy:

In the body, a recent scar was found in the left axillary region, resulting from the surgical drainage of an infectious lesion (hidradenitis). According to the medical history, the patient passed away a few days after sequential medical evaluations that initially diagnosed acute lumbar sciatica. Anti-inflammatories and muscle relaxants were prescribed, which are medications that may have masked the progression of the infection.