Rashford made it five games without a goal in 2021 as Premier League leader United played out a goalless draw with champion Liverpool at Anfield on Monday (AEDT).
Nevertheless, the England forward is enjoying a productive season with 14 goals and six assists in all competitions.
The 23-year-old has only bettered that goal tally once across the course of an entire campaign, with 22 last term bolstered by six conversions from eight Premier League penalties - duties that now lie with Bruno Fernandes.
He netted one spot kick in the 4-1 Champions League win over Istanbul Basaksehir this time around and his 12 open-play goals in Europe's elite competition and the Premier League came from shots with a cumulative xG value of eight - highlighting some impressive efficiency in front of goal.
"Marcus is getting to an age where he’s maturing and will come to his prime soon," Robson told Stats Perform News.
"I think he’s really improved over the last two years. He's worked really hard on his physique and fitness. But I do believe there’s another 5-10 percent in Marcus.
"When things get easy for him in games he becomes a little bit lazy on the ball and gives it away more than he should.
"But that’s probably because we know he can create with a great pass, and sometimes when you're that little bit younger you don't have that real strong mentality to go further.
"I think he can go on to be a great player for Man United and England."
Although Anthony Martial opened the scoring in the 2-1 New Year's Day victory over Aston Villa, Rashford's fellow forward has found the going tougher in front of goal overall this season.
The France international has five goals in 22 games, with just two in 14 in the Premier League.
Robson believes the 25-year-old can sometimes struggle for confidence, while also highlighting his heavy workload as United's 2019-20 schedule ran on until the Europa League semi-final defeat to Sevilla in August.
"It was a long season for a few of the lads," he said. "It took a few of them quite a time to get going, they had no pre-season after Sevilla loss.
"I think it affected [Martial]. I don't think he's a totally confident lad.
"I think he needs encouraging, but I'm sure as the season goes on he just needs a run of games with a couple of goals."