Philbin said it was a combination of factors since the team first gathered in April that led to the decision, which he said what in the best interest of the organization.
Tannehill, the eighth overall pick in the 2012 draft, won the competition against returning starter Matt Moore and veteran David Garrard, whose chances to win the job disappeared when he had to undergo knee surgery the day after the preseason opener.
Barring an unforeseen change, Tannehill will become the first rookie quarterback in Dolphins history to start an opener.
Tannehill and Moore both addressed the media on Monday, but their press conferences came both Philbin made the announcement.
Philbin said he told the quarterbacks before practice, saying Moore took the news in a professional manner while Tannehill was excited.
Neither quarterback revealed anything when they spoke to the media, but Moore did say at one point he was looking forward to a decision being made.
Unfortunately for him, that decision didn't go his way.