Mourinho has been touted as a potential successor to United boss Sir Alex Ferguson.
Perhaps that is why Charlton claims Mourinho "sometimes tries to sell himself".
But Sir Bobby, recognised as England's most famous footballer, told the Daily Star Sunday: "Alex respects people in the game if they have made a contribution and rightly so - and Mourinho has.
"There's no way I can put the record straight on Mourinho because I don't really know him that well.
"I will leave all that to Alex. What I know is that Alex is the greatest manager that there has ever been.
"Who am I to say what Mourinho is going to do? I don't know - I suppose it takes all sorts."