That goes for preseason games too. They can't trust McClain, the eighth pick in the 2010 draft. Not yet.
Maybe midway through the season, or at the end of the season, he'll have earned the Cowboys' trust.
This isn't so much about the 18-day jail sentence he received in an Alabama courtroom after being found guilty on two misdemeanor charges of disorderly conduct and resisting arrest.
It's more about him retiring twice in the past few months.
Football is a difficult, emotional profession that leaves players with aching joints and broken bodies. That's the cost of the adrenaline rush that comes from the adulation of millions, a fat bank account and the camaraderie of the locker room. That said, this game is too physically demanding to persuade anyone to play. Each player must supply his own passion for the game. The 6-foot-4, 259-pound McClain says he's regained that passion. He said his 3-year-old son wanted him to play again, and he said Jerry Jones calling him long-distance from Turkey after the trade with Baltimore inspired him to play too.
You have to wonder if that's going to be enough to make McClain prepare to be his best each and every day.
"My life wasn't going in the right path," McClain said of his retirements. "Being a young father of two, I had to sit down and realize I have things I need to do to be a better man for those two little boys.
"The best thing for me to do is get away from football and work on my personal life. I told Mr. [Ozzie] Newsome, when I felt like I had that clear I would come back. Unfortunately, when I did come back, everything wasn't settled. At the end of the day, I still have to stay true to my word and true to my family."
Garrett must carefully guard his emotions because the Cowboys are vulnerable. He must let the videotape from practices and preseason games -- not desperation -- dictate whether McClain is worthy of a roster spot.
It won't be easy. After all, the Cowboys are so desperate for a middle linebacker that they traded for a guy who didn't even play last year. It's a low-risk deal because the Ravens won't be compensated if McClain, scheduled to earn $700,000 this season, doesn't make the team.
Durant will try hard and work hard, but he's never been a difference-maker in the NFL. Rookie Anthony Hitchens, drafted in the fourth round to be Lee's backup, is still learning the nuances of the NFL.
The Cowboys want him starting only as a last resort. And second-year linebacker DeVonte Holloman is still making the adjustment from college safety and outside linebacker to NFL middle linebacker.
McClain has significant playmaker potential, but no coach in a contract year wants him starting. This, friends, is why it's going to be so easy for Garrett and the Cowboys to fall in love with McClain.
Hey, it has already started, if you think about it.
Garrett spent all of the past week telling us he didn't want the players to run a conditioning test, and he didn't want them in full pads the first two days of practice, to ensure their bodies had done some football movement before they started practicing with pads.
The goal was to limit soft tissue injuries. Well, McClain hopped off a plane late Friday night after being sentenced in Alabama, and he was on the field Saturday morning.
"I really thought they might ease him in, might need to since he hadn't had OTAs, hadn't had the kind of drills," Jerry Jones said. "The deal with him, and because of the issue retiring, what you want to see is what you were seeing when he wasn't in the drill. He was back over there. His body language was great."
But will it stay that way?
When the training camp grind gets real and his body is sore, will he still want to play? When a 350-pound guard pancakes him and he hasn't see the smile on his children's faces for a few weeks, will he still want to play?
The first day went well.
Linebacker coach Matt Eberflus said McClain picked up the system quickly, though the Cowboys limited his participation in some team work. His conditioning seemed OK, and he certainly didn't mind hitting. He tackled a running back and took him to the ground on his first play, which isn't supposed to happen.
It was more of the same on Day 2.
"We have such a need with our linebacker, and here you got a guy that has shown up enough to be drafted in the first round and has shown he was certainly a top college player, and he's shown flashes of that while he's been in the pros," Jones said. "That's good opportunity when you're in this business to be able to go get a guy that has that kind of skill and those kinds of issues. You don't get them if they're free of issues."
It's way too early to tell if McClain has handled all of his issues. Until the Cowboys know for sure, they must temper their enthusiasm.