Both players were in tight position battles this preseason but listed as starters on the team's official Week 1 depth chart.
Tunsil was selected No. 13 overall by the Dolphins in May; his draft stock fell when a video was released via social media of him smoking from a bong through a gas mask. Tunsil spent all of spring and most of training camp on the second team. But the rookie continued to improve and started at left guard in Miami's second and third preseason games.
"I think Tunsil did earn it," Gase said. "We made it hard on him. Obviously, [the media] talked about it so much. But he earned being in that spot."
Foster joined the Dolphins a week before training camp and quickly made up ground against Jay Ajayi. Foster, who changed his number Monday from No. 34 to No. 29, consistently showed veteran savvy in the passing and running game during practices and limited reps in the preseason.
Ajayi didn't help his case by fumbling and dropping a pass in last week's preseason finale against the Tennessee Titans.
"The reason is experience," Gase said Monday of choosing Foster. "But at the same time, I think Arian, from the time he's gotten here, has really shown that is the correct spot for him to be in. He had a really good camp. He did everything we asked him to do, as far as what we wanted to see in the preseason, and he's been very consistent."