In general, it is easy to plug a new player into the lineup during the season. Unlike other sports, there are fewer signals or plays to learn, as baseball still comes down to hitting, catching, and throwing the ball. Typically, the biggest concerns come down to clubhouse chemistry and whether or not that player is a fit.
However, catcher is not the average position. A catcher needs to learn the pitching staff, understand their stuff, and learn how to get through when they are struggling. They need to be a leader, and have to know how to get through the opposing lineup.