JUPITER, Fla. — Adam Wainwright remembers precisely where he was on that sad December morning when the best hitter of his generation left the St. Louis Cardinals.

“I was on the golf course, on hole No. 4 at Frederica Golf Club at home when I found out that he had signed with the Angels,” said Wainwright, who lives in St. Simons Island, Ga. “That was a life-altering, shifting moment, how big that was.”

That was in 2011, just after Albert Pujols had won his second championship with the Cardinals. A day like Monday, when Pujols signed a $2.5 million deal to return to St. Louis for a farewell season, seemed impossible. He was 31 years old when the Angels gave him $240 million to play for them for the next decade, plus a personal services contract for another decade after his retirement. It looked like the permanent shedding of a perfect fit.

“Was it a sad day, a dark day?” John Mozeliak, the Cardinals’ president of baseball operations, said on Monday morning. “Yes, for sure. But in this industry, especially in baseball, you have to keep thinking ahead. You have to be looking forward.”

And so both parties forged ahead and apart, with neither tasting a title again. Without the Cardinals, Pujols made just two trips to the postseason. Without Pujols, the Cardinals made seven but reached only one World Series, a six-game loss to Boston in 2013.

Pujols collected milestones along the way: his 500th and 600th home runs and his 3,000th hit. He is the only player to reach those benchmarks in a career with at least two championships.

The departure from St. Louis marked a clear dividing line, though, and Pujols’s worst season with the Cardinals was better than his best for the Angels. Yet because he kept his family foundation in St. Louis, Pujols always felt connected.

“I felt like, yes, I might be wearing a different uniform, but I felt like I never left,” he said, adding that he often returned for charity events. “The people still treat me the same way, whether I was wearing the Cardinal uniform, whether I was wearing the Angels or the Dodgers, they never change. That’s what’s so special about the Cardinals’ fans. They love you when you wear their uniform, but they still love you because you’re part of this organization.”

Now — as the oldest player in the majors, at 42 — Pujols is officially back, largely because of his performance after the Angels released him last May. Pujols joined the Dodgers and played in 85 games, hitting .254 with 12 home runs. Overall, he clobbered left-handers for a .603 slugging percentage. With the designated hitter in the National League now, Pujols can make an impact.

“Oh, he’s got something left,” said Oliver Marmol, the Cardinals’ first-year manager. “He’s got more than something left. Albert wants to play this year because he can help a team win.”

The Cardinals, who lost to Pujols’s Dodgers in the N.L. wild-card game last October, will open the season without two of their better young pitchers, the right-handers Jack Flaherty and Alex Reyes, who have shoulder injuries. But the team signed two free agent pitchers — the left-hander Steven Matz and the right-hander Drew VerHagen, who revived his career in Japan — and still has catcher Yadier Molina, 39, guiding the staff.

Molina and Pujols have said definitively that this will be their final season, and Wainwright, 41, said he was leaning that way, too. All three are royalty in St. Louis, with Pujols bringing an especially regal air to an organization that prides itself on tradition.

“This organization never closed the door on me, and I never closed the door on the organization, either,” Pujols said. “It’s just a great opportunity.”

Molina and Wainwright had been eager to reunite with Pujols, pestering him regularly on FaceTime about his plans. When Pujols arrived at the team’s spring training complex on Monday, he found Wainwright napping before his start and woke him with a bear hug. During the game, the fans roared as Pujols strode from the right field corner to the Cardinals’ dugout. He smiled so he wouldn’t cry.

“I’m a pretty emotional guy,” Pujols said. “It doesn’t take that long.”

Pujols played 76 games at first base last season but understands he will not unseat the Cardinals’ Paul Goldschmidt, a four-time Gold Glove winner and one of the league’s most reliable sluggers. Pujols said he would do whatever Marmol wanted, and hoped to mentor young players the way teammates like Placido Polanco and Edgar Renteria did for him as a rookie in 2001.

Wainwright sees it happening already. After his outing on Monday, he noticed some younger players dressing to leave for the day. When those players saw Pujols and Molina heading for the batting cage, Wainwright said, they changed back into workout clothes and tagged along.

Pujols can offer intangible help like that, through his guidance and example. But he did not sign to be an ambassador. Pujols is returning with a purpose.

“Anytime Albert’s motivated,” Wainwright said, “it’s a very, very dangerous thing.”