SAN DIEGO — BYU could be without two of its senior leaders today when the Cougars visit San Diego State.

Running back Manase Tonga (knee) and free safety Scott Johnson (ankle) have not practiced much this week, and a decision on playing them will be made at game time.

Both players fill important leadership roles for the Cougars' offense and defense, respectively.

This week, Mendenhall called Johnson "our most valuable player defensively right now, not in stature, not necessarily by all the plays he is making, but by how he is contributing and helping the other 10 defensive players play. He is very smart, very bright, has a great idea of what we are trying to do, and why, and he just lends a lot of confidence to the rest of his teammates."

Last week, in BYU's 59-21 rout of UNLV, Johnson recorded the first two interceptions of his career. He did not play in BYU's only loss of the season, against Florida State, due to a concussion.

"When we play really good defense here at BYU, it is tied directly to the free safety," Mendenhall said. "And (Johnson) really has more impact on the outcome of a game as any of us coaches do, on that day. He is making decisions in real time, and he is coordinating the defense, basically, on the run while he is out there."

If Johnson can't play, he'll be replaced by true freshman Craig Bills.

BYU quarterback Max Hall said this about Tonga last week: "We are a totally different football team with No. 11 on the field. I don't think there is any question about that. Manase brings an energy, an excitement and a physical (mentality) to our offense."

If Tonga can't play, sophomore Bryan Kariya will likely play in his spot.

NO GO FOR SAN DIEGO: BYU defensive lineman Russell Tialavea, who has missed the past few weeks with a torn MCL in his knee, was hoping to be able to return to his hometown of San Diego for today's game.

But on Thursday, it was determined he wouldn't travel to San Diego.

Tialavea, who has been called to serve a mission to Chile, is set to go into the Missionary Training Center on Dec. 30. Though he's listed as a senior, Tialavea was able to get back a year of eligibility due to a previous injury. He said he plans to play one more year after he returns from his mission.

HOKE ON MENDENHALL: Twenty years ago, first-year San Diego State coach Brady Hoke was hired as Oregon State's defensive line coach. Mendenhall was a graduate assistant working under Hoke.

Hoke said this week he "cherishes" those memories of his time with Mendenhall. He added, "There's no doubt that you knew he would be a tremendous football coach because of the passion he has for young men and the passion he has for the game. He did a tremendous job for me as a defensive line coach. He was a guy who played in the secondary, so he really did a tremendous job. ... He's continued that."

TOPS IN THIRD-DOWNS: BYU leads the nation in third-down conversions. The Cougars are converting 61.97 percent (43-of-81) of their third-down opportunities.

e-mail: jeffc@desnews.com