Rooney has come under increasing criticism for his performances this season.
And Mourinho dropped the England captain to the bench for Saturday's impressive 4-1 win over Leicester.
But Robson, who made 461 appearances for United, believes the striker should still be one of the first names on the team sheet.
Robson told the Manchester Evening News: “When United are having a bad time it’s always Wayne that gets the criticism. He’s always the easy target.
“He sets very high standards and if he’s not scoring or creating goals and we’re losing straight away people look at him.
“The fans always seem to pick him out.
“I don’t think he’s been our worst player. There are a lot of other players who have not performed this season who could easily come in for more criticism than Wayne.
“He’s not been our best player and he’s definitely not hit top form yet. But at this moment in time he would still be in my United starting XI.
“Whether that be up the park or playing just that little deeper."
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