Marcus Rashford has been challenged to become a ’20 or 30-goal-a-season’ striker by an Old Trafford legend.The Manchester United forward turns 21 on Wednesday, and former Red Devils captain Bryan Robson wants to see him progress further following his remarkable rise to prominence over the last three years.Robson believes Mancunian Rashford, who has scored 33 times in 133 appearances since making his debut under Louis van Gaal in 2016, can continue to grow by adding more goals to his game. Article continues below Editors’ Picks Man Utd ready to spend big on Sancho and Haaland in January Who is Marcus Thuram? Lilian’s son who is top of the Bundesliga with Borussia Monchengladbach Brazil, beware! Messi and Argentina out for revenge after Copa controversy Best player in MLS? Zlatan wasn’t even the best player in LA! “I think you always want to improve and better yourself,” Robson told the club’s official website . “Marcus is in a great position – he’s been in the first-team squad for over three years now.“You can see him maturing, he’s building his stamina and strength, and he’s looking really good and sharp at the moment. He’s going in the right direction.“The only thing I would like to see is his finishing just being that little bit better.”Rashford has played largely on the wings under Jose Mourinho, though the United boss used him as a central striker against Everton on Sunday after dropping his go-to centre-forward Romelu Lukaku.But Robson wants to get to a point “where we get Marcus up to something like 20 or 30 goals in a season, whether he’s playing out wide or through the middle.”The former England captain added: “He’s adapted really well, because he has been playing wide as a player for quite some time now – he hasn’t had too many chances through the middle as a centre-forward.“But I think it educates you as a player and Marcus is definitely going the right way to be a top-class player for Manchester United over the years ahead.”Rashford burst on to the United scene with two goals against Midtjylland on his debut as an 18-year-old substitute in the Europa League in February 2016 and has collected FA Cup, League Cup and Europa Cup winners medals, as well as representing England at the European Championships and World Cup, in his short career.“Everything that Marcus has done has been very good so far, but it’s always the same with young lads – at the age of 21, you’re still a young lad – it’s about improving and getting better and better,” said Robson.Mourinho has been accused in some quarters of not turning to Rashford often enough.But the United manager said recently: “For Marcus Rashford, that means a player, I think, with more appearances [than any team-mate] in the past two years. “I think he’s a privileged player, a privileged kid or, if you want to say now, a man, that has had so many opportunities to develop himself and he’s going to keep having them.”Rashford and United are next in action on Saturday as they travel to sixth-placed Bournemouth in the Premier League.