I would say that Carrasco is a better bet. I don't think Lyon, Person, or anyone else Boston has right now is ultimately going to be a viable closer. Eventually, they will realize the stupidity of their approach and make a trade to bring in a real closer (Benitez, Baez, Julio, Alfonseca, Williamson are likely candidates)
MacDougal lately has lost velocity on his fastball, and he has always had terrible control. Unless they can rapidly fix his control problem, he won't likely be able to hang onto the closer's role too much longer. If MacDougal needs to be removed, Carrasco is the likely hier apparent, although it's also possible Grimsley will get the call. Neither of these guys are really great bets right now, and I would probably rather grab a great middle reliever (Dotel, Rhodes, Santana, Hawkins, Hasegawa) than one of those two.