SAN DIEGO -- Merrill Kelly threw a belt-high fastball to Fernando Tatis Jr. in the sixth inning Monday night. Tatis -- with that simple, yet violent right-handed swing the Padres have dreamed on for years -- turned on it with authority. The baseball soared deep into the left-field seats, and Petco Park rose in unison.

Won't be the last time.

Tatis, MLB Pipeline’s No. 2 overall prospect, left no doubt with his first career big league home run in the Padres’ 10-3 series-opening loss to the D-backs at Petco Park. He jumped on Kelly's first-pitch heater, sending it a projected 395 feet toward the Western Metal Building with a 107 mph exit velocity.

“Got that one out of the way,” said Tatis, who last week became the youngest player to start on Opening Day in 20 years. “First one, here we go. A lot more to come.”

In Padres history, only Roberto Alomar has homered at a younger age than Tatis’ 20 years and 89 days. Alomar was four days younger than Tatis when he went deep against the Pirates on April 30, 1988.

San Diego trailed 9-1 at the time, and Tatis' homer, which plated Manny Machado, only cut the deficit to 9-3. In a way, that might be the nature of the Padres this season. They’ll have some growing pains to get through with one of the youngest rosters in the sport. But they're quickly becoming must-see TV, capable of wowing at any moment with any number of exciting players.

“I know he’d like to hit a big home run when it counted for us and helped the team win a baseball game,” Padres manager Andy Green said. “It’s still a great swing, and always a good feeling to get your first big league home run.”

Since his surprise promotion to the Opening Day roster, Tatis is 4-for-18 after a 1-for-4 showing against the D-backs. But he’s impacted the game in other ways -- namely with his speed and his defense. He also made an absurd diving play on a David Peralta grounder in the fifth inning Monday night, but Peralta beat the throw for an infield hit.

After the game, the fan who retrieved Tatis’ home run met the rookie shortstop on the field and presented him with the baseball. The two took pictures together, and Tatis autographed a different ball, before retreating to the clubhouse. He plans to present the baseball to his father, the longtime big league infielder he grew up watching.

“There's a lot of memories, seeing my dad all my life and watching him hit balls out of the field,” Tatis said. “Now, history has been changed. Now he's seeing me. It's very special.”

Strahm roughed up
The Padres acquired Matt Strahm at the 2017 Trade Deadline, envisioning that Strahm would one day occupy a place in their rotation. They knew he was coming off major knee surgery, and they knew it might be a while before that plan came to fruition. But they felt Strahm’s stuff would make the wait worth it.

Strahm’s rotation transition, it seems, is still a work in progress. In his first outing as a full-time starter for the Padres, Strahm was roughed up for five runs and eight hits over 2 2/3 innings. The 27-year-old left-hander was hit hard from the outset, allowing four extra-base hits -- including a leadoff homer to D-backs right fielder Adam Jones.

“I felt like I had it; I just didn’t have full conviction behind it,” Strahm said. “Whether that’s me thinking out there too much, trying to place it, trying to be too fine with it … I’ve just got to be myself when I’m out there and be aggressive.”

Strahm was a dominant relief weapon for the Padres last season, posting a 2.05 ERA with 10.1 strikeouts per nine innings. He made five starts in 2018, but those were merely as the first pitcher in a line of relievers on scheduled bullpen days. He never went more than 3 1/3 innings.

During the offseason, Strahm bulked up significantly and strengthened his left leg specifically. He stretched to five innings during Spring Training, and he handled the workload well. That wasn’t his issue on Monday night.

“Just go throw it,” Strahm said. “I’m not a place pitcher. I’m not going to nibble. That’s not me. That’s when I get in trouble. That’s what happened today.”

Perhaps most concerning was a dip in Strahm’s velocity. His four-seam fastball averaged 90.9 mph, 2.4 mph slower than last season. There was always going to be a dip upon his transition to the rotation. But Strahm’s Spring Training velocity was slightly higher than it was Monday, too.

“It just wasn’t a good night,” said Green. “I thought the first couple pitches had conviction on them. When Adam Jones turned that fastball around, it just didn’t have as much conviction on the heater after that. … It was one of those days where it went sideways early on, and he wasn’t able to recover.”

Tidbits
• In the scheme of the game, it didn’t mean much, but Robert Stock worked a pretty spectacular ninth inning. He threw 12 pitches, nine of them strikes, and recorded three swinging strikeouts.

Stock spiked an 0-2 fastball to D-backs reliever Jon Duplantier that clocked at 100.6 mph -- the fastest pitch he’s thrown and the second-fastest pitch Statcast has ever recorded for a Padre. Only a Jose Dominguez 100.8 mph heater in July 2016 was thrown harder.

• Eric Hosmer went 2-for-4 on with a double on Monday, raising his average to .353. He has split time in the No. 2 spot with Wil Myers. Evidently, it’s nice to be hitting ahead of Manny Machado. The Padres’ No. 2 hitters have combined to go 10-for-17 with three doubles and a homer this season.

• After Strahm and Robbie Erlin struggled, Phil Maton helped keep the rest of the ’pen fresh. He worked a career-high three innings on a career-high 43 pitches, allowing one run on three hits.