The Tigers made the moves before playing at Philadelphia on Tuesday night.
Rookie Spencer Turnbull started the series opener against the Phillies instead of Ross.
Ross (1-3, 4.03) had been scheduled to start. Turnbull (1-2, 2.77) is making his sixth start of the season, and the rookie is looking for his second career victory after earning the win in the nightcap of a doubleheader April 23 against Boston.
Turnbull hasn't allowed a run in 11 innings, and he is second among AL rookies with 26 strikeouts.
Mercer, who started against the Phillies, had been out because of a right quad strain. He is batting .233 in 13 games.
Harrison had been on the 10-day injured list retroactive to April 27 with a bruised left shoulder. He is hitting .156 with a homer and seven RBI in 22 games.