They are saying that despite the minor league deal Rosario will likely be the Nats starting LF. Either way, neither Pham nor Rosario were being promised the ABs they wanted by the Phillies. Any of them would have taken the $8 million Merrifield got (we know now). Merrifield was the one who agreed to be a bench player, and won't take an inordinate number off starts away from Rojas. Rosario is obviously at least counting on playing enough for the Nats to get his full incentives.
The Phillies may be enamored with the fact that Merrifield can play multiple positions, and overpaid for it, but he really can't/shouldn't. He's barely ever played 3B and hits the same side as Bohm and Sosa, he's not a SS, Stott shouldn't be sitting that often against LHP and Harper won't take many days off. They signed him to be a LF and PH, not because the roster really needed more flexibility. Last year that was Josh Harrison's spot.
And nearly all DHs are just poor fielding LFs or 1Bs to begin with. Schwarber can play either of those positions (even if he'll never do the latter again). And if he had to stand in RF he probably could (Matt Stairs used to).
Pham probably still needs to find his deal and ABs on a bad team. He can join the Phillies on July 31. Or Rosario could.