"We're not sure yet," said offensive coordinator Brandon Doman.
Lark, who threw six touchdown passes in his only career start on Nov. 24, took most of the reps in practice Monday because Nelson is still not completely recovered from the rib cartilage injury that sidelined him against the Aggies.
Coach Bronco Mendenhall said more than two weeks ago that Nelson would probably start against the Aztecs, but wouldn't say that again Monday because Nelson is not completely healthy.
"So at this point we have to plan on James working [more] and going forward, and then just wait and see where Riley catches up," Mendenhall said. "We've got time, but Riley is not healthy enough right now to be considered the starter yet."
As for Nelson, he acknowledged his rib cage is still "tender and sore" and that he hasn't been fully cleared to play. He said he did get a few reps Monday.
However, he is "almost positive" that he will be 100 percent by game time, and expects to start.
"Extremely important," Nelson said, when asked how important it is to him to play. "I haven't gone through what I have gone through to sputter out towards the end. I really want to finish, and I want to finish strong."
Doman said it will be Mendenhall's call regarding who will start, and he believes coaches will have a better idea in a few days after watching Nelson closely.
"We are just trying to get a feel for who gives us the best chance to win. It's really that simple," Doman said.
Mendenhall did have an answer to a question that has been on the minds of BYU fans everywhere since his name surfaced for the opening at Colorado 10 days ago. That job was filled Monday by San Jose State coach Mike MacIntyre.
Mendenhall said when he was asked about the CU job last Wednesday by a reporter in San Diego, he had not been contacted by the school. But after Cincinnati coach Butch Jones decided to not take the job (he has since taken the Tennessee job), Colorado did contact him, Mendenhall acknowledged.
"There was interested expressed, but there was no interview," he said.
Asked if he is being selective regarding the job openings he is interested in, Mendenhall said: "I would say definitely that selective would be too mild of a word. I am really happy here."
The coach has one year left on the three-year contract he signed in January of 2011.