Lopez opted to sign with the Red Sox for the remainder of the season instead of joining the Padres for the pennant stretch.
Lopez, placed on release waivers by St. Louis on Tuesday, hit .231 with seven home runs and 36 RBIs in 109 games. He played 58 games at third base and 24 each at second and shortstop.
Had he joined the Padres, he would have provided infield depth for the final 10 games of the regular season as well as given the team another left-handed bat off the bench. Lopez is a switch-hitter.
Because it is past the Sept. 1 deadline for postseason eligibility, Lopez would not have been able to play for the Padres if they reached the postseason.
The Padres lost utilityman Jerry Hairston for the rest of the season earlier this week, when he was diagnosed with a stress fracture in his right tibia. The team has been exploring additions since the injury was disclosed.
Lopez, 30, was let go by the Cardinals a day after showing up late for a game against the Marlins. St. Louis general manager John Mozeliak told reporters that this wasn't the first incident of tardiness for Lopez.
Lopez is a career .266 hitter over parts of 10 seasons in the Major Leagues, having played for the Blue Jays, Reds, Nationals, D-backs, Brewers and the Cardinals twice.
Lopez signed a one-year deal with the Cardinals in the offseason worth $1 million. He can be a free agent after the season.