Here are some advanced stats for last season (from basketball-reference.com):
Player A: .177 WS/48, 3.0 BPM, 3.3 VORP
Player B: .152, 2.8, 2.8
Player C: .158, 3.0, 2.3
Player A is All-Star Isaiah Thomas. Player B is Jae Crowder. Player C is Amir Johnson. (For comparison, Al Horford was at .172, 4.1, 4.1)
A lot of trade ideas seem to treat Johnson as a nearly worthless throw-in who is only there to match salaries. Some posters seemed to think he was an easily replaceable player who was likely to be cut before his contract became guaranteed. In reality, Amir Johnson was arguably about as valuable as Jae Crowder and both were close to IT, so the Celtics starting lineup might consist of a top 20 player in the league, an All-Star, two players who give you almost the same production as the All-Star, and an All-Defensive 1st-Teamer.