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.