LeBlanc, a 31-year-old southpaw, was acquired by trade from the Blue Jays on Wednesday to help bolster an injury-plagued rotation and got Friday's start in place of Taijuan Walker, who is dealing with tendinitis in the arch of his right foot.
LeBlanc was 7-2 with a 1.71 ERA in 14 starts for Triple-A Buffalo this season and pitched in Japan last year.
With starters Felix Hernandez and Wade Miley on the 15-day disabled list and Walker's next start delayed by his foot issue, LeBlanc provides a temporary hand at least. The club also had to pull Sampson from his start on Thursday before he even threw a pitch after the rookie right-hander felt discomfort in his elbow during his final warmup tosses.
Miley is starting a rehab game for Class-A Everett on Friday night and is expected to rejoin the rotation on Wednesday against the Pirates if all goes well. Walker will test his foot this weekend and the plan is for him to be able to return to the rotation on Thursday, without needing a DL stint.
LeBlanc was leading the International League in ERA (1.71) and innings pitched (89 2/3), and he was third in strikeouts (85). Over his final seven starts, he was 5-1 with a 1.20 ERA, 56 strikeouts and 15 walks in 60 innings.
LeBlanc is a veteran of seven Major League seasons and has a career record of 21-33 with a 4.47 ERA in 107 games, including 71 starts with the Padres (2008-11), Marlins ('12-13), Astros ('13), Angels ('14) and Yankees ('14). He was a second-round Draft pick of the Padres in 2006 out of the University of Alabama.
LeBlanc's last MLB appearance was at Safeco Field on Sept. 28, 2014, when he threw two scoreless innings of relief in a 4-1 loss for the Angels. His last big-league start was Sept. 23, 2014, when he threw 5 1/3 shutout innings with five hits in a 2-0 victory at Oakland.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.