Spoiler alert for Mission 46 The Man Who Sold The World.

I know the ending of MGSV that Venom Snake is not the real Big Boss. So now my question is that was the real Big Boss ever in a coma ? If so how did they both wake up at the same time ? If Big Boss was not in a coma then why was Venom Snake in a coma? Or was the coma of Venom Snake induced so that Big Boss can fully recover?

In addition, what was that piece of metal lodged in his skull ?

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2 Answers

Big Boss and Venom (the medic) were both in the helicopter crash at the end of Ground Zeroes.

Big Boss goes into a coma and Venom is placed into a medically induced one as a result of the crash also. Big Boss wakes up before Venom and puts his whole "Venom is the new Big Boss" plan into place.

When Venom wakes up, that is where The Phantom Pain game begins.

The 'Metal' lodges in his skull is bone fragment from Paz who exploded in the helicopter - This is explained by the doctor in the opening scene where he is showing you the x-rays.

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"Punished "Venom" Snake, also known as Big Boss, and formerly known as Ahab, V, Big Boss's phantom, or simply Snake, was a mercenary commander, formerly a combat medic for Militaires Sans Frontières (MSF), who led Diamond Dogs and Outer Heaven as one of Big Boss' most trusted lieutenants.

During his time as a medic, Venom Snake saved Big Boss's life during the destruction of MSF in 1975 by shielding him from an oncoming explosion, losing his left arm and the use of his right eye in the process. Despite his sacrifice, Big Boss fell into a coma. Venom Snake was then placed into an artificially induced coma, undergoing hypnotherapy and plastic surgery so that he could act as a body double for Big Boss. This was done in order to protect Big Boss from XOF forces who had destroyed MSF nine years earlier."

Got this from this link

So basically, he was the doctor who helped in retrieving the explosives from Paz's insides during the helicopter scene, and was induced in to a coma as the boss already was in one (which was not induced), in order to protect him.

And also, about the piece of metal, Venom Snake got it from the helipcopter crash, on the end of the game there is a cut-scene that explains that.