The 18-year-old striker is in Roy Hodgson's 23-man squad for Euro2016 and despite having played just once for the Three Lions prior, is settling in nicely with the other players.
Defender Smalling, who is rooming with Rashford, told reporters: "Marcus was lucky enough to know some of them because of some of the younger ages and they're around his age anyway, but to start with it's nice that he's got a couple of familiar faces who he knows.
"Now you can see he's a lot more relaxed.
"You can see him on the training pitch, he's smiling, because it's an easy squad to get to know."
England kick off their Euro2016 campaign against Russia in Marseille on Saturday.