ST. LOUIS -- As a general rule, baseball managers prefer to work with a player for a season before feeling like they truly know them. Seeing how they handle success and failure, how they push through adversity and how they respond in clutch situations are often the determinants managers use before forming opinions on the long-term potential of a player.

Regarding Cardinals’ rookie Jordan Walker, manager Oliver Marmol doesn’t need the final two weeks of this season to make up his mind about the overflowing talent and massive, raw potential of the 21-year-old slugger. As if Marmol’s mind wasn’t already made up about the 6-foot-6, 245-pound outfielder, Walker offered up another reminder of the promising trajectory his career seems to be on when he smashed a tiebreaking, 400-foot home run that helped the Cardinals beat the Phillies, 6-5 on Sunday at Busch Stadium.

“He’s definitely on a path to be great and he’s going to help us win for a long time,” Marmol said after Walker turned on a 98.4 mph sinker from Philadelphia’s Seranthony Domínguez to break a 5-5 tie. “His skill set, matched with his aptitude and overall personality, lend themselves to him continuing to develop well. He cares and he wants it.”

A day after becoming the fourth-youngest player in the rich history of the Cardinals to register 100 career hits, Walker drilled the first game-winning home run of his young career. Earlier this season, he hit go-ahead homers against the Marlins (July 5, top of the ninth) and Pirates (Sept. 2, bottom of the seventh), but the bullpen squandered those leads he provided. On Sunday, he drilled his 16th home run of the season in the eighth, and closer Ryan Helsley made it stand up with a save.

As for some insight into the high standard Walker has for himself, he came into the game-deciding at-bat in the eighth angry with himself for narrowly missing when he faced Phillies starter Taijuan Walker. He did not miss against Domínguez.

“I was a little frustrated with myself because those are pitches [from Taijuan Walker] that I feel like I should hit, and I ended up just getting under them and just missing,” Walker recalled. “I ended up talking to [Cards hitting coach] Turner [Ward] about it, and he told me I was pulling off a little bit. I went back into that next at-bat and tried to make that adjustment, and it ended up working out.”

Not only did Walker win Sunday’s game with his bat, but he also opened it by showing off the massive strides he’s made with his glove in right field with a diving catch of a 115.2 mph laser off the bat of Kyle Schwarber. For months, Walker -- a converted outfielder who previously played third base -- has been one of the club’s first players on the field before games to get in extra work with assistant coach/Cardinals legend Willie McGee. He said his outfield growth could be measured in what would have likely happened had that same ball been hit to him months earlier.

“Absolutely no chance [he makes that catch], I don’t think so,” said Walker, who covered 34 feet on the catch. “I feel a lot more comfortable now than before after working with Willie. Before, my read would have been slower, and my route would have been worse. With Willie, I’m giving him all the credit because he’s always pulling me out there, and I’m ready to work with him.”

Walker’s rare combination of talent, baseball IQ and a drive to improve have the Cardinals believing that he can be a core piece of their lineup going forward for years to come. Star slugger Paul Goldschmidt, who also homered on Sunday and helped the Cardinals avoid the Phillies sweeping them in the season series for the first time in franchise history, marvels at the tools Walker brings at such a young age.

“This whole year, seeing his growth as one of the youngest players in the big leagues, it’s been really good,” Goldschmidt said. “I’m sure he’d say there’s been a lot he’s learned this year, but big credit to him. He’s done a great job.”

Minutes after Sunday’s win, Walker said it was too soon to think about where his first game-winning homer ranked among the memories he’s made in his first MLB season. However, he did admit that the reality of this first season has greatly exceeded the dreams he had coming in.

“Oh, it’s more than I imagined, man,” he said. “Going out there and playing in front of those fans, I can’t describe that feeling. It’s much more than I ever imagined, for sure.”