ST. LOUIS -- The St. Louis Cardinals factored in Jhonny Peralta's drug suspension when they negotiated a four-year deal worth more than $50 million with the free-agent shortstop.

General manager John Mozeliak said Monday that Peralta had admitted wrongdoing and served his penalty and that the Cardinals weren't about to appoint themselves morality "police." Mozeliak said the club was confident that this was an isolated circumstance.

"Character and makeup are something we weigh into our decision-making," Mozeliak said. "In his case, he admitted what he did, he took responsibility for it.

"I feel like he has paid for his mistakes, and obviously if he were to make another one, then it would be a huge disappointment."

The 31-year-old Peralta was suspended for 50 games last summer following Major League Baseball's investigation of the Biogenesis of America anti-aging clinic, which was accused of distributing banned performance-enhancing drugs to players.

Mozeliak agreed with critics that penalties aren't strict enough.

"You do need a deterrent, and right now 50 games does not seem to be necessarily stopping it," Mozeliak said.

But he also believed attitudes have changed.

"I think Major League Baseball has done a great job trying to clean up this game, and I feel like they've taken great steps," he said.

Mozeliak said in a statement Sunday that the Cardinals were "pleased" to sign Peralta, but the move drew a different reaction from some big leaguers.

Arizona Diamondbacks reliever Brad Ziegler, the team's union player representative, tweeted that "it pays to cheat" and chided owners for "encouraging PED use."

Although he did not specifically name Peralta, Ziegler expressed his opinions in a series of tweets.

People really don't understand how this works. We thought 50 games would be a deterrent. Obviously it's not. So we are working on it again. — Brad Ziegler (@BradZiegler) November 24, 2013

Just trying to make our game better when I leave it than it was when I got into it. Don't have all the answers, but trying, & MLBPA knows... — Brad Ziegler (@BradZiegler) November 24, 2013

Free-agent pitcher David Aardsma also took to Twitter:

Apparently getting suspended for PED's means you get a raise. What's stopping anyone from doing it? #weneedtomakeachange — David Aardsma (@TheDA53) November 24, 2013

I had 2 major surgeries in 5 months and made it back clean, nothing pisses me off more than guys that cheat and get raises for doing so — David Aardsma (@TheDA53) November 24, 2013

Peralta underwent a physical Sunday, though he did not attend the introductory news conference.

St. Louis also explored a trade but wasn't willing to part with several prospects. Mozeliak said negotiations went quickly after groundwork was laid at the GM meetings.

Peralta and Stephen Drew were the top two shortstops in free agency. Peralta was a plus for the Cardinals because they've got a lot of left-handed hitters and he bats right-handed.

Mozeliak said rookie Pete Kozma, the starter last year who could end up back in the minors next season, was a better defensive shortstop than Peralta. The Cardinals prized Peralta, an All-Star two of the last three seasons, for his bat.

Peralta batted .303 with 11 homers and 55 RBIs in 107 games this past season and four times has hit 20 or more homers. Kozma hit .217 with one homer and 35 RBIs.

Mozeliak said he'd be satisfied if the Cardinals stood pat the rest of the offseason but added the team will listen to offers. St. Louis filled its other need last week by trading third baseman David Freese to the Los Angeles Angels for center fielder Peter Bourjos.

Coming off a second World Series appearance in three seasons, the Cardinals are loaded with young pitching and the lineup is set.

Matt Carpenter is set to move from second base to third base next season to make room for rookie Kolten Wong, a first-round draft pick in 2011. Matt Adams will start at first base with Allen Craig moving to right field.

The Detroit Tigers did not extend a qualifying offer to Peralta, meaning there would be no compensation draft pick involved in his move from the AL Central champions to the NL Central winners.

Information from The Associated Press was used in this report.