Neymar scored with a right-footed shot in the 65th minute as Brazil struggled to get near the Belarus goal despite controlling possession at Old Trafford.
Belarus had taken the lead when Brazilian-born playmaker Renan Bressan scored in the eighth minute, but Alexandre Pato tied it up with a header in the 15th after a cross by Neymar. The Brazil star also set up Oscar's final goal in injury time.
The victory gives Brazil six points in Group C, three more than Belarus.
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