Yes, I should probably disclose that I played American football at college (technically Canadian analogue), and at 18 I was 6'4, 195 pounds, and at 21 I was 215 and ran a 4.68 40, with about a 9-11% body fat. So, I'm pretty well versed in what it takes to put on muscle.
Rashford is a very skinny mesomorph, who is probably trending slightly on the ectomorph side of things. With the nutrition he can afford, and the time and effort he could put into filling out his frame, he could put that 20 pounds comfortably in 12 months, but I'd suggest he do it over 2-3 years for performance reasons. I added 20 pounds of lean muscle over 3 years, but I was also pretty big when I was 18 and when you've already added a lot of muscle, the gains become obviously much more difficult.