Yes, this strikes me as a big worry. Reuben's first game defending at centre, with Tui just back playing on a different side with someone he's never played outside of. Add the defensive prowess of Johns and Tuilagi, and it could get ugly on our left.
On the flipside, if Garrick and Tui play well together, this could be our strongest backline going forward (with the inclusions of a fit Koula, and Schuster)
Positive is a strong bench and pack with an unchanged right side.
But if Turbo is not 100% he should not play!