Below are Barrett 2014 stats through 5 games and Jones stats through 5 games in 2015. Keep in mind, Barrett did not have a seasoned line and running back in the first 5 games of 2014.

Barrett
88-133 passing 66%
1,354 yards passing
17 TD to 5 Int
276 yards rushing

Jones
63-106 passing 59%
867 yards passing
5 TD to 5 Int
127 yards rushing

I think Urban is starting Jones because he wants Barrett to stick around 2 more years. If Barrett start this year he'll more than likely leave after the 2016 season. If he sits this year he will more than likely play 2 more years. This is the only way I see him not starting.