CREVE COEUR — Despite reports and every indication this spring that the San Francisco Giants had taken a shine to reliever Luis Perdomo and planned to keep him in the major-league bullpen, the former St. Louis Cardinals farmhand was designated for assignment late Saturday night.

But his return to the Cardinals is hardly a guarantee.

Perdomo was the Giants selection in the Rule 5 draft, plucked from the Cardinals’ Class AA bullpen. To retain control of the righthander the Giants had to keep Perdomo on their major-league roster for the entire season, ala outfielder Brian Barton with the Cardinals last season. By designating Perdomo for assignment — read the reports here and here and finally here — the Giants begin the process that could result in Perdomo’s return to the Cardinals. It’s just not a given.

Perdomo must pass through waivers, and the other 28 teams will have their chance to take him. The Rule 5 procedures will apply to that team as well. The San Diego Padres, for example, are in desperate need of relievers. (Newly acquired Luke Gregerson was a candidate entering the weekend for the big-league roster. Landing Perdomo would be the Padres third Cardinals minor-league reliever that they acquired since last season.) And if San Diego does snag Perdomo he must remain on their roster all season, per Rule 5 standards, for them to gain complete control of his rights.

Only when Perdomo clears waivers must he then be offered back to the Cardinals.

The Cardinals are the only team that can pass Perdomo to the minors without the other teams having their chance to take him.

Perdomo went 2-1 with a 6.75 ERA in 13 appearances this spring for the Giants. In his 12 innings, he struc k out 10 and walked eight. His early appearances were impressive, and the Giants, according to reports out of their camp, had eyes on keeping him in the bullpen. In his last appearance of spring, Perdomo uncorked two wild pitches that led to a loss. Wrote MLB.com:

Perdomo, 24, owned a 3.48 ERA through 10 Cactus League appearances. But his lack of command in his final three outings unnerved the Giants. In that stretch, he yielded five earned runs, allowed nine hits and walked three in 1 2/3 innings.

The Cardinals acquired Perdomo from Cleveland in 2008 for Anthony Reyes. This past winter, the Cardinals elected not to protect Perdomo on the 40-man roster — despite having room to do so. Instead, the opted to protect Matt Scherer, who missed his exposure to major-league camp because of injury, and to keep several spots open on the 40-man roster. The Cardinals would dodge some embarrassment if Perdomo slips back to them and likely into the Class AAA bullpen.

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