Of course, that didn't mean the former Scottsdale Saguaro High School standout couldn't handle the job. He made that quite clear before a sold out crowd Saturday at Sun Devil Stadium.
"I really wasn't surprised," offensive coordinator Mike Norvell said.
Said receiver Jaelen Strong: "We knew all along. It wasn't nothing new to us. We knew all along D.J. would come in and make plays. That's what he does. He makes plays."
Foster made plenty of plays, rushing for a career-high 124 yards and two touchdowns in ASU's 58-21 win against the rival Arizona Wildcats.
His 23 attempts, another career-high, were two more than he had in his previous five games combined.
"I'm a really big believer in patience," Foster said Tuesday. "Wherever I can help out the offense, at the end of the day, that's what I try to do. It's a team game, but it felt good to just be definitely back in the back field and get those carries and just help my team win."
Foster, who rushed for more than 3,000 yards while leading Saguaro to the 2011 Division III State Championship, had been used more as a slot receiver. His 54 catches for 550 yards are second-most on the team.
Foster's role changed, however, when Grice hurt his left leg late in the Pac-12 South-clinching win at UCLA. Offensive coordinator Mike Norvell said Foster "had a different look in his eye" at practice last week thanks to the opportunity in front of him.
"I just had a different look in my eye because I just didn't want my teammates to feel like something was going to slack," Foster said. "Our offensive line and our offense and this team has put in so much work. I wanted them to feel that there was really going to be no difference and no step back ... I was going to take care of my side."
He'll be looked to again this week when ASU hosts Stanford in the Pac-12 Championship Game Saturday. Grice will miss his second straight contest, though he may be ready to return by ASU's bowl game.
A Sun Devil victory Saturday would send ASU to Pasadena, Calif., and its third-ever Rose Bowl appearance.
"It's kind of surreal," Foster said. "We've been talking about it so long. We're right here. We're right where we need to be. It came by fast. We put a lot of hard work into it, but nothing matters if we don't finish strong."