After being named to the Padres' Opening Day roster in 2018, Lamet injured the ulnar collateral ligament (UCL) in the elbow of his throwing arm. He underwent Tommy John surgery and missed the entire 2018 season.[9] In 2019, he rehabbed for Lake Elsinore and El Paso before being activated from the injured list on July 4.[10] In 14 starts, he recorded an ERA of 4.07 with a record of 3–5. He struck out 105 batters in 73 innings.[11] In the pandemic shortened 2020 season, Lamet finished with a 3–1 record and a 2.09 ERA, in 12 starts.[12] However, he was unavailable for the playoffs due to a bicep injury.[13]
After spending the beginning of the 2021 season recovering from a UCL sprain, Lamet made his season debut as the starting pitcher against the Milwaukee Brewers on April 21. However, he was removed from the start after two innings and 29 pitches after experiencing right forearm tightness.[14] Lamet pitched in 22 games in the 2021 season, only nine of which were starts. He recorded a 4.40 ERA and 57 strikeouts in 47 innings.[15] In 2022, he worked entirely out of the bullpen, pitching 12+1⁄3 innings over 13 games with a 9.49 ERA.[16]
In 2023, Lamet made 16 appearances (four starts) for Colorado, but struggled to an 11.57 ERA with 31 strikeouts in 25+2⁄3 innings pitched.[16] He was designated for assignment on June 17[20] and released on June 22.[21]
Boston Red Sox
On June 28, 2023, Lamet signed a minor-league contract with the Boston Red Sox organization[22] and was assigned to the Triple–A Worcester Red Sox where he posted a 3.72 ERA with 16 strikeouts in 19+1⁄3 innings pitched over three games.[23] On August 7, the Red Sox selected his contract and added him to their major-league roster, with the intent of using him as a reliever.[24] Lamet's struggles in the major leagues continued, as he made his Red Sox debut the next day against the Kansas City Royals, pitching two innings, giving up three runs and striking out one. He was designated for assignment the next day,[25] and later sent outright to Worcester on August 12.[26] However, on August 14, Lamet rejected the assignment in favor of free agency.[27]
Los Angeles Dodgers
On February 3, 2024, Lamet signed a minor league contract with the Los Angeles Dodgers[28] and was assigned to the Triple–A Oklahoma City Baseball Club to start the season.[29] However, the Dodgers recalled him to the major leagues on April 1, before he had appeared in any minor league games.[30] On April 3, Lamet picked up his first career save against the San Francisco Giants. In three appearances for Los Angeles, he logged a 2.08 ERA with three strikeouts across 4+1⁄3 innings. Lamet was designated for assignment by the Dodgers on April 6[31] and outrighted to Oklahoma City on April 8.[32] He pitched in nine games in the minors, with a 3–1 record and 4.82 ERA.[4] Lamet triggered an opt–out clause in his contract and elected free agency on June 4.[33]
Kansas City Royals
On July 19, 2024, Lamet signed a minor league contract with the Kansas City Royals organization.[34] On November 6, he elected free agency.[35]