We are only nine games through the season, so any stats involving team lineups need to come with a disclaimer due to the small sample size. However, the Suns' starting lineup of Bledsoe, Knight, Tucker, Chandler and Morris has played 127 minutes together and boasts an offensive rating of 110.5 and a defensive rating of 95.5. That net rating of +15.0 is among the best in the NBA.
Replace Tucker with Warren and you get a much different result. The four starters plus Warren have logged just 23 minutes together through six games, but that makes them the 3rd-most commonly used Phoenix lineup. And the results so far have been disastrous, as that unit has a net rating of -28.6 and an effective field-goal percentage of just 40.2 percent.
As long as the Suns continue to have such an effective starting lineup, there's no reason for a change. I think most would agree that T.J. Warren is playing better than P.J. Tucker and that he has a very promising future. But due to match up reasons it simply makes more sense to keep Tucker in the starting lineup until it stops being an effective strategy. In the meantime, coach Hornacek can continue to reward Warren with more minutes, perhaps getting to the point where Warren's playing time will actually eclipse Tucker's. Warren can also be given more of a taste of playing time in clutch situations, but Tucker should still be the go-to option at small forward in key moments.
http://www.brightsideofthesun.com/2015/11/15/9739186/p-j-tucker-vs-t-j-warren-analyzing-the-suns-small-forward-battle-two