Millar agreed to a $2.1 million, one-year contract with the Orioles on Thursday.
Young made his major-league debut in September after he was recalled from Triple-A Ottawa. He hit .303 with one homer and three RBI in 14 games with Baltimore.
Young batted .288 with 13 home runs and 81 RBI in 123 games for Ottawa. The Orioles have 10 days to trade, release or send him outright to the minors.