McTominay left United last summer for Napoli and finished the season a Scudetto winner and with the Serie A Player of the Year award.
Scotland captain Robertson said on Thursday: "I think he went over there with a point to prove.
"He wanted to prove Man Utd wrong and prove people in the Premier League wrong, and he’s gone over there and done that pretty successfully.
"That’s credit to him, credit to his attitude and his determination to be the best version of himself.
"Now he’s a king over there. At Man Utd he probably lacked getting that run of games consistently, he was in and out and things like that."
McTominay deserved better
The left-back also said, "He probably didn’t get the respect he deserves.
"Then he goes over there, puts in a magnificent season and, fair play to him, he ends it with the league title.
"Everyone knows how passionate their fans are so, for them to love one of ours, it’s pretty special for Scotland, too.
"They love Billy (Gilmour) too, so credit to the both of them for going over there, being successful and winning the trophy."