Why? Because McDonald, the Trojans' star senior safety, has watched every single snap his brother, Tevin, has played for UCLA this season, and Tevin has been one of the Bruins' mainstays as a sophomore safety.
But USC's McDonald doesn't plan on giving his coaches a scouting report on the other McDonald.
And why's that?
"That's not my position to tell them," T.J. McDonald said Wednesday. "But he's not gonna tell them about me, either. They can kind of figure that out. I'm not gonna say what he does well and what he doesn't."
But doesn't he know some of his younger brother's strengths that he could relay to the offensive coaches? And, perhaps more importantly, some of his weaknesses?
"I do," McDonald said. "But that's not my job. My job, and what I do, is between the lines and my defense."
Both McDonalds have emerged into future-NFL safeties at this level, although T.J. has emerged as the clear leader in head-to-head matchups. Last season, he had 10 tackles and an interception against the Bruins, while Tevin had just two tackles.
This year, the brothers' numbers are more even: T.J. has a team-high 83 tackles, two interceptions and four passes defensed, and Tevin has 62 tackles, one interception and seven passes defensed, plus a forced fumble and two fumble recoveries.
And the brothers' teams are quite even, at No. 17 and No. 18 in the BCS standings and within five spots in the AP and coaches' polls.
In what may be the last time the two oppose each other on a football field, USC's McDonald said he realizes what 's on the line Saturday.
"There's not more at stake this time, but it is my last go-around. And if we win, I can say he's never beat me," McDonald said. "That is big, and I'll appreciate that further down the road, too."
Their father, Tim, is in his first season as the secondary coach at Fresno State. The Bulldogs have a bye this week, and the elder McDonald plans to be in Pasadena for Saturday's game.