Rashford has joined Villa from Manchester United on-loan with an option to buy.
Walcott told BBC Sport: "I think sometimes as a player, a change of voice and scenery, even getting out of the city, experiencing being on your own, away from distractions and just focussing on the ultimate goal which is playing and performing at the level we all know.
"We all want to see the smile, I want to see it back, I want to see the levels that he has I can relate to the fact that at times, being at a club for such a long time, do that step. You have got to take that leap.
"I’m saying to Marcus Rashford now be the first one at training, be that last one off the pitch. When things are going well be in that gym, be seen doing the right things because there’s loads of eyes around all these clubs these days, they always keep an eye on you, so I’d just advise him, even if it’s really hard to do, for me I think it’s going to be a really positive move, I really do.
"It’s a different environment and one he will thrive in. Manchester United haven’t been doing great without him."