The 20-year-old was one of the main talking points for England as they lost 2-1 to Spain in their Nations League opener at Wembley Stadium on Saturday night.
Despite scoring the Three Lions’ goal from United team-mate Luke Shaw’s cross, Rashford missed a number of opportunities to draw Southgate’s side level.
Rashford has started just one of United’s four Premier League matches so far this season, having been sent off in their 2-0 win at Burnley before the international break.
And when he has played under Jose Mourinho, Romelu Lukaku’s presence up top has seen Rashford shunted into a wide role rather than occupying a central position.
Rashford has aspirations to play centrally for club and country but having yet to cement a regular starting spot, has been warned by Southgate that he needs more game-time to improve.
The England manager did however back the front-man – who has 32 goals in 126 appearances for United – to come good in the long run.
Southgate told reporters: “The only way for him to improve is by playing. He knows we have real belief in him.
“I think we also have to remember his age and what he’s achieved in a short period of time.
“He’s got a tremendous mentality, he’s got great talent, we think he’s got everything to go on and be a top player and I’m sure he’ll do that.”
“I’d rather focus on the fact he got his goal, which was really well taken. Good save from the header. And he’s in the right position to try and finish the next one.
“As he gets more experience and confidence in those situations, he’ll finish them.
“He’s a player that in this period we definitely wanted to invest time in – that’s why we started [Joe] Gomez and Rashford and Shaw.
“He’s scored a goal and was in the penalty box more. At times, his previous goals have come outside the box.
“We know he can hit things, we’ve been encouraging him to get higher up the pitch, not drop quite as deep and he got his reward for that.”
More likely than a suggestion he may need to move away from United, Southgate’s comments are simply a plea for critics not to be overly harsh on Rashford and a reminder that he is still in the early days of his career.
But ex-England No 9 Alan Shearer believes a move away may be necessary if Rashford wants to secure a week-in, week-out role where he starts in his favoured central position.
The all-time top Premier League goalscorerer said: “As long as Romelu Lukaku stays fit he is not going to get a starting spot as the lone striker at Old Trafford.
“As long as Jose Mourinho is the manager, there is not going to be two up top.
“Even if Lukaku did not play, my money would be on Alexis Sanchez being moved into the middle and further forward.
“And would Mourinho really turn to Rashford if he needed another striker? Or would he go to the transfer market for a tried and trusted ready-made goalscorer?
“Rashford has been played so long in a wide position in a three, that it does take time to readjust to his natural position.
“Time is not a great commodity in football nowadays. I am not advocating Rashford moves away from Old Trafford. It is a great club to be at.
“But if he wants to achieve an ambition to be recognised as a great goalscorer it is just not going to happen there.
“He would need to move with the assurance that his next club will make him the main man, the striker, the one up top or one of two. Then I truly believe he would develop into a goalscorer.”