Brad Pitt can't always lean on Angelina Jolie when he's in pain -- which might explain why he's using a cane!

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The 48-year-old actor stepped out in Beverly Hills Thursday with the aid of a walking stick. "He tore his MCL," a source tells Just Jared, explaining that Pitt injured one of four ligaments that stabilizes the knee joint.

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The cause of the World War Z actor's injury is not yet known -- nor is the reason he chose to wear an all black jumpsuit.

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As of now, Pitt is still expected to walk the red carpet at the 69th Annual Golden Globes on January 15. The Tree of Life actor is nominated for Best Actor in a Motion Picture Drama alongside George Clooney (The Descendants), Leonardo DiCaprio (J. Edgar), Michael Fassbender (Shame) and Ryan Gosling (The Ides of March).

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