ST. LOUIS -- Seconds before he stepped into the box in a tie game with the bases loaded in the bottom of the seventh inning of his MLB debut Thursday afternoon, Cardinals rookie Jordan Walker looked down to third and got a simple, but blunt message from teammate Willson Contreras.

"He looked at me and just told me to breathe," Walker recalled after he had his first MLB hit and his first MLB RBI in the Cardinals' 10-9 loss to the Blue Jays at Busch Stadium. "I looked right at [Contreras] after walking up from the on-deck circle and he said, 'Breathe.' After that, I simplified, put something in play and beat it out to first base. That's what happened, and it felt good to get a run for my team."

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Walker, the No. 4-ranked prospect in baseball per MLB Pipeline, was playing in front of 47,649 fans, approximately 25 friends and family members and 13 Cardinals Hall of Famers. Still seven weeks shy of his 21st birthday, Walker (20 years and 312 days) became the youngest Cardinal to make his MLB debut since Rick Ankiel (20 years, 35 days) on Aug. 23, 1999, and the youngest position player to debut since David Green (20 years, 274 days) on Sept. 4, 1981.

When Walker smashed a line-drive single with an exit velocity of 103.4 mph up the middle in his first MLB at-bat, he became the youngest American-born player in MLB to record a hit since Bryce Harper (20 years and 312 days) on Aug. 24, 2013. Upon rounding first base, Walker clapped his hands together and waved in the direction of the Cardinals dugout.

Walker, who made the team despite never playing at Triple-A, said the baseball from his first hit will go to his father, Derek, who was among those in attendance at Busch Stadium on Thursday.

"I was just more pumped up than anything," Walker said of the second-inning single. "My dad knows what to do [with the ball from his first hit]. Who knows? He probably does [already have a spot picked out for the ball]."

Nerves did get the best of the top rookie from time to time on Thursday. After receiving a standing ovation from the crowd, Walker nearly forgot to go shake the hands of the 13 Cardinals Hall of Famers in attendance at the game. Later, he had a ball carom off the wall and go between his legs and he twice overthrew cutoff men on throws in from right field.

Overall, he said the day will be one he'll remember.

"Every time I walked into the outfield, seeing everybody filled up in the stands, it really just kind of hit me that this is where I want to be and this is where I want to play," he said.