Start Watson now! Make Watson sit!... Jeez. Wait for training camp and let the talent sort itself out - "Ball don't lie." If you cannot wait for training camp and preseason for the coaches to choose the starter, consider the options:
1. Play Watson now. If he does poorly learning under fire, the Texans will suck and the Browns will be the big winner (they have the Texans first two draft picks in 2018). Throwing a rookie to the NFL wolves has a long history of producing much less than stellar first seasons. Meanwhile, the Texans risk premature mental, physical and quite possibly career damage to their new QB.
2. Let Watson sit (a season), then start him in 2018 (if the coaches think he is ready). Watson gets the chance to maintain a healthy mind and body while he learns the system and the NFL. If he does poorly for any reason, the Texans will at least have the benefit of their own (higher) draft picks.
In conclusion, there is less risk for the Texans on the field, for the coaches, and for the front office to wait and hope "Ball don't lie" to sort this all out by the start of the regular season. If they still can't decide who to start, there is less risk for the Texans on the field, for the coaches, and for the front office to go with Option 2 and have Watson sit for 2017. If a starter other than Watson does well in 2017, no QB conroversy. If a starter other than Watson falls on his face, no controversy starting Watson in 2018.