At the start of the 2008–09 season, Treacy was called up to the first team under the new management of Paul Ince. He then made his league debut for Blackburn Rovers in the opening game of the season, in a 3–2 win over Everton on 16 August 2008. Treacy found his playing time in the league, mostly coming from the substitute bench. At times, he was given starts in a number of matches for the side. Treacy went on to make twelve appearance throughout the 2008–09 season. Reflecting on his performance, local newspaper Lancashire Telegraph commented about the player, saying: "Next season will be an important season for the young Irishman after a disappointing campaign this time round. A hot prospect as he came through Rovers’ youth ranks, has never really transformed that into first team ability. A direct and powerful runner, who has again showed his potential, but will need to deliver in earnest next season."