The decision to hold off bringing Tressel on board stems from NCAA infractions at Ohio State that led to players being suspended.
"After the announcement of Coach Jim Tressel's agreement to join the Colts as a game-day consultant, questions were raised with respect to the equity of his appointment as opposed to suspensions being served this season by present and former Ohio State players," Colts Vice Chairman Bill Polian said in a statement.
"Over the weekend Coach Tressel, [team owner] Mr. [Jim] Irsay, Coach [Jim] Caldwell and I had a discussion of the issue. In addition, we had a conversation with league officials to apprise them of the details of Coach Tressel's employment and the issues we were reviewing.
"At Coach Tressel's suggestion, and with Mr. Irsay's concurrence and support, we have decided to begin Coach Tressel's employment effective with our seventh regular season game. We have informed the league office of our decision and expect that they will be supportive of it. We are very happy Coach Tressel will be joining us."
Tressel, who resigned at Ohio State in May, is to review replays for the Colts.