Mourinho has been criticised for not playing the 20-year-old, who scored twice for England during the global break, enough in United's opening four Premier League games.
"We have to remember that Harry Kane at that age had hardly played anywhere near the matches for Spurs", he added, emphasising that Rashford is yet to reach his full potential and will continue to improve.
"It's not my business to interfere with club managers, they have a hard job to do and they have big squads at the top six, huge competition for places". He had 53 appearances but if you want to take to the minutes of play, he played 3,068 and if you want to divide that by 90 minutes of the matches, he played 34.2 matches of 90 minutes.
'In 2017-18, 35 Premier League matches, eight in Champions League, five in FA Cup, three League Cup matches and the European Super Cup final. He played a total of 52 matches with 2,676 minutes.
"Marcus Rashford is not Dominic Solanke".
Ruben Loftus-Cheek is another of England's squad that advanced to the semi-finals of the World Cup struggling for minutes at Chelsea.
The problem for Rashford, and Southgate, is that he is not getting to play in his best position at United.
United entered the worldwide break with a 2-0 win against Burnley but will face unbeaten Watford at Vicarage Road on Saturday in another hard test for the Red Devils.
"Speaking about Marcus, I think I can expect that Sunday I'm going to be highly criticised for not playing him", he said.
"When he's mad at me I know he is mad at me, and I try to do what he wants so he is happy again".
A move to another club could provide more playing time but sticking at United will help him win silverware. But for the Manchester United supporters is important they have the right idea of how things are in reality.
Rashford subsequently went to Euro 2016 as England's boy wonder but only played a cameo role in that tournament and was then largely a substitute at this summer's World Cup.
"Since I was here, he was never one day out of selection because of my decision".
Manchester United striker Romelu Lukaku defended Jose Mourinho's confrontational style of management, saying that the Portuguese deserves respect for not hiding his true feelings like several Premier League bosses.
In his news conference before Saturday's league match at Watford (15:00 BST), Mourinho outlined his use of Rashford over the past two seasons.