Selvig knows Thor, who is in space, and doesn't own a phone, Nat, who is dead, Tony, who is dead, Steve, who is retired, Clint, who is retired, Nick Fury, who is a skrull, and Banner, who is injured. So, say Darcy overcomes her ego and decided she is out of her depth. She phones Selvig and manages to get through despite the time zone difference. Selvig phones Banner, and manages to get through, despite the time zone difference. Selvig manages to persuade Banner that he is a) needed (even though he isn't), and b) wants to get involved (even though Banner dislikes fighting and knows nothing about magic). Banner hops on a Greyhound bus (because he is too big for a moped now, and couldn't steer anyway with his arm in a sling), and 24 hours later arrives in Westview. Where he finds there is nothing he can science and nothing he can smash.