PITTSBURGH -- With only two left-handed pitchers on their 40-man roster, the Pirates have been busy adding lefty rotation and bullpen candidates over the last few days.

After agreeing to a Minor League deal with southpaw Robbie Erlin earlier this week, the Pirates on Friday signed veteran lefty Derek Holland to a Minor League deal that includes an invitation to Major League Spring Training. The Pirates announced the signings of Holland and Erlin on Friday, along with non-roster deals for catcher Andrew Susac and outfielder Charles Tilson. The Bucs also announced the signing of free-agent infielder JT Riddle. The lefty-hitting Riddle, 28, has hit .229 across 223 Major League games, playing shortstop and center field.

Holland will have a chance to make the Pirates’ rotation or bullpen out of Spring Training. He can opt out of his contract and become a free agent at the end of Spring Training if he’s not guaranteed a spot on the Opening Day roster, according to an industry source. USA Today reported that Erlin has a similar opt-out clause in his contract.

Holland, 33, has been linked to the Pirates in the past. There was mutual interest between him and the club after the 2016 offseason, when he wound up signing with the White Sox. He first hinted at the agreement on Thursday night, posting an image on Twitter of a guinea pig wearing a pirate hat.

Holland was a dependable starter as recently as 2018, when he posted a 3.57 ERA and 3.87 FIP in 171 1/3 innings for the Giants. But he took a giant step backward and moved to the bullpen last year, recording a 6.08 ERA and 1.51 WHIP while allowing 20 homers in 84 1/3 innings over 51 appearances, including eight starts, for the Giants and Cubs.

Holland remained tough on left-handed hitters last season, holding them to a .192/.286/.242 slash line in 114 plate appearances. The only home run he gave up to a lefty last year, in fact, was a walk-off grand slam Bryce Harper hit in Philadelphia. Meanwhile, right-handed hitters slashed .288/.385/.612 against him.

Holland, who grew up less than three hours away from PNC Park in Newark, Ohio, came to fame with the Rangers in the early 2010s. The lefty finished 16-5 with a 3.95 ERA and four shutouts in 198 innings over 32 starts in 2011 and went on to pitch in the World Series for the second straight year. His best season came in ‘13 with Texas as he went 10-9 with a 3.42 ERA and 189 strikeouts in 213 innings over 33 starts.

The Pirates have been scouring the market recently for rotation depth. Beyond their top six starting options -- Joe Musgrove, Trevor Williams, Chris Archer, Mitch Keller, Steven Brault and Chad Kuhl -- they have potential Triple-A starters JT Brubaker and Cody Ponce on the 40-man roster. Their non-roster starting options include James Marvel, Hector Noesi and now, Holland.

Pittsburgh learned the value of rotation depth last season when the club had to scramble for starters and briefly experiment with the opener concept following a rash of injuries. The Pirates used 14 different pitchers to start or open games last season, and only three of them -- Musgrove (31), Williams (26) and Archer (23) -- made more than 20 starts.

Holland and Erlin also have a path to the Opening Day roster through the bullpen. If Brault cracks the rotation, the Pirates’ other lefty relief options are Sam Howard, Miguel Del Pozo, Williams Jerez, Nik Turley and Blake Weiman. Howard is the only pitcher in that group with a spot on the club’s 40-man roster.

Susac, 29, has bounced around the league a bit since the Giants selected him in the second round of the 2011 Draft, 37 picks after they selected Kyle Crick. The catcher has a career .221/.283/.373 slash line with seven homers, 23 walks and 94 strikeouts in 300 plate appearances over 113 Major League games with the Giants, Brewers and Orioles.

Susac only played 26 games for the Royals’ Triple-A Omaha affiliate last season before being placed on the injured list on May 26 with a broken left wrist that sidelined him for the rest of the year. He will join Luke Maile and fellow non-roster invitee John Ryan Murphy in the competition to back up catcher Jacob Stallings. Pittsburgh previously invited non-roster catchers Jason Delay and Christian Kelley to Spring Training, and those two should serve as depth in Triple-A Indianapolis or Double-A Altoona to start the season.

Tilson, 27, was the Cardinals’ second-round pick in the 2011 Draft. He has played in 95 games for the White Sox over the last two years, slashing .244/.309/.288 with one homer and six steals. He performed better in Triple-A last season, hitting .288/.345/.398 for Chicago’s affiliate in Charlotte. He spent time at all three outfield positions last year, making him a versatile depth option if he begins the season in Triple-A for the Pirates.