Who that will be is still up for grabs.
"You'll know on game day. That'll be about the last time I answer that," Wells said. "Those two guys are going to work together. We're going to see who settles in the best, and whoever it is, that's who we're going with."
Wells also announced running back Joe Hill is out for the year with an ACL tear, leaving the Aggies three offensive starters down in just their past two games.
The injuries particularly Keeton's have changed the context of a game initially billed as a battle for first place in the Mountain Division and a conference championship berth. Boise State, a banner program for non-BCS autobid conferences, opened the week as a 7-point favorite over Utah State, which struggled to a 31-14 loss against BYU after Keeton went down.
For the Aggies, however, the game represents a chance to re-establish and reinvent themselves. Although they've encountered some admittedly devastating losses, Wells and his players expressed confidence they'll be able to compete with arguably their toughest remaining opponent this week.
"My first season, we were 4-8 and a lot of people felt that we were doing a lot better," tight end D.J. Tialavea said. "Obviously now we're 3-3 and it's very disappointing. It's something that we've kind of changed in this program, the we expect to win every single game. We expect to fight to the end and not have any big-time losses."
Utah State is coming off its worst loss since 2010, and bouncing back without Keeton will be challenging, to say the least. Keeton was one of the most prolific passers in the country, and was among the top 25 leaders in passing touchdowns, total offense and passing efficiency before the BYU game.
Harrison, a former starter at Snow College, stepped in for the final three quarters last week and finished 18-for-41 with 185 yards and a passing touchdown. Garretson, who signed with the Aggies last spring, has not yet played in a college game, nor taken many reps since fall camp.
But the strong-armed freshman will have a chance to seize the starting role this week, starting with a 50-50 split between the two in reps on Monday. Wells said he would make the call early in the week, then hammer out a game plan in closed practices leading up to the game.
Wells said he was encouraged by the way his team kept playing hard against BYU, if not its best. Sini Tauauve'a fit in well on the offensive line, and overall, Wells said, he thought the unit played well.
However, in a Keeton-less offense, Utah State will have to find a way to improve on "playing well" in those areas. And the Aggies will need to improve what wasn't so great against BYU as well.
The margin for error just got thinner.
"We're going to have to rally around some new guys," Wells said. "It's going to be next man up. We're going to have to play just a little bit better."
Boise Stateat Utah State
P Saturday, 6 p.m.
TV • CBS Sports Network