~183 days in a season, roughly.
172 service time days = a service time year per the CBA.
Pompey has 88 service time days so far.
IF they can get lucky with Ceciliani, or Lake, or Brown, or health generally, and limit Pompey's service time to less than 84 days this year, then they can gain an extra year of control. Worth a shot? Of course, he'd end up a super 2 player in this scenario and they'd have to go to arbitration with him before 2019 (and then three more times).
The ideal scenario in sending him down has him developing in AAA and the MLB team doing just fine without him. Then they can call him up early July for almost three full months of developed Dalton, supplementing a World Series run. Or, alternatively, they have a mature, developed trade chip that still has a full slate of team control. But if the 4th OF isn't cutting it then they can just call on Pompey.