(You are just what the doc ordered)
“Just what the doctor ordered” is an idiom meaning “precisely what is needed”.
Here it refers to Dr Dre, How Dre needed someone like him to mentor, and he was just the right guy for it. In other words he’s just what Doctor Dre wanted/needed.
Also, can be interpreted as what the doctor prescribed. Eminem is the drug the rap game needed and Dre is to thank for introducing him into it.
Note: This line is highly similar to the slogan that Dr. Dre used for the release of his Beats By Dre Pill portable speakers. Eminem has promoted the brand in videos before so it’s not that far of a reach to say he wasn’t inspired by the brand for this line.
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