Demetrious "Mighty Mouse" Johnson says Conor McGregor needs to think about a career change if he beats Floyd Mayweather ... telling TMZ Sports the boxing money is too good to stay in the Octagon.

We got Johnson out in L.A. and asked him who he was rolling with in the super fight ... and if he ever thinks we'll see Conor back in the UFC.

DJ said he's got Conor by knockout, and once that happens McGregor should focus on fighting the winner of the GGG/Canelo fight, and not ever grace the Octagon again.

Johnson's logic? $100 million payday Conor's getting in the ring ain't happening in the UFC, so why go back?

But McGregor's gotta win first.