Baseball’s bible says never talk to a pitcher during a no-hitter. It can be true for debuts too. The Rockies tried not to trip up the pitcher performing for the first time at Coors Field.

So everybody stayed out of Jon Gray’s way.

After the 24-year-old rookie’s first inning Tuesday, second baseman DJ LeMahieu sprinted to the dugout stairs and stopped. He stepped aside and waited for Gray to walk off the mound. Then he patted Gray on the back without a word.

WATCH: Jon Gray and Walt Weiss on Gray’s debut

Gray was on his way. The No. 3 pick overall in 2013 out of the University of Oklahoma became the third pitcher from the first round of his draft class to compete in the major leagues. He threw four up-and-down innings against the Seattle Mariners. He exited off the hook in a 3-3 game. But Seattle pulled away with seven runs in the final four innings for a 10-4 win.

“Nobody can ever say you didn’t make the big leagues,” said Nick Hundley, Gray’s catcher.

The Rockies made a game of Gray’s first start, but the Mariners slugged their way to the runaway victory behind Robinson Cano’s three RBIs.

PHOTOS: Colorado Rockies debut Jon Gray against Seattle Mariners

Hours before Gray threw his first pitch for a team that has placed so much of its future on his back, he sat for a chitchat in manager Walt Weiss’ office.

“He seemed at ease,” Weiss said. “But you try to make it as normal as possible. Because we all know it’s not normal.”

Gray’s first few months in the majors will be anything but normal. Already nearing his career high for innings pitched in a season, the Rockies will cap Gray with a tight pitch limit and an innings ceiling.

In the first inning against Seattle, Gray threw 33 pitches to six batters. To lead off, he threw three 96 mph fastballs and two 95 mph fastballs to Austin Jackson before the Mariners center fielder lined out to LeMahieu at second base.

Gray then walked Kyle Seager, struck out Nelson Cruz, and gave up an RBI double to Robinson Cano and a run-scoring single to Jesus Montero.

Pitching coach Steve Foster jogged to the mound. Hundley too. Can you say something to a rookie to dilute the tension?

“Yeah,” Hundley said, “but it’s probably not for the cameras.”

Gray struck out the next batter, Mark Trumbo, on six pitches.

“It really took me a while to settle down,” Gray said. “I was a little disappointed in the first inning, but proud how I bounced back.”

Colorado starter Chad Bettis knows debuts. He gave up five runs and two home runs in his first start, at Atlanta in 2013. Two years later, he’s a Rockies regular.

“For me, you get to see how fast they get the butterflies out,” Bettis said of how he’ll scout Gray as a rookie pitcher. “Maybe they have a horrible start. I know I did. But once you get that first start, you get into a routine — and understand that you’re up here for a reason: because everybody believes in you.”

The key for a Rockies rookie pitcher — really any pitcher tasked with throwing at altitude in the most exhausting pitcher’s park in baseball — is to dodge the cannonballs. Seattle ace Felix Hernandez allowed 11 hits in winning Monday night at Coors Field.

Hundley examined his pitcher. The veteran catcher wanted to grade Gray’s presence on the mound, he said.

“Out there, you have to expect to get people out,” Hundley said. “The guys who do that are very successful.”

Gray’s second inning spiked with improvement. He threw just 10 pitches in a 1-2-3 frame. In the fourth, Trumbo singled off Gray, and pitcher Vidal Nuño singled in a run. Four innings were enough.

Gray gave up three runs (two earned) on five hits. He struck out four and walked two. And 43 of his 77 pitches were strikes.

“There was a lot going on,” Gray said of coming up to the big club. “It was pretty crazy the past couple days. But once it’s game time, you have to lock in.”

The Rockies bolstered Gray’s outing. Charlie Blackmon led off the first with a solo home run. Hundley in the fourth slammed a two-run homer to center.

And Rockies rookie Kyle Parker — a 2010 first-round pick — hit his first career home run, an opposite-field job off Nuño in the sixth.

But with Rockies relievers Rafael Betancourt and Boone Logan handling the eighth inning, the Mariners scored three, capped by Nelson Cruz’s two-run single, to put the game away.

“There was a lot going on,” Gray said. “It was pretty crazy the past couple days. But once it’s game time, you have to lock in.”

Nick Groke: ngroke@denverpost.com or twitter.com/nickgroke