I'd like to get some MLB innings before that UCL blows out.
They don't have to add him to the MLB roster until the end of next year, so there's a judgment call about when to start his clock and options. If you think there's a chance that he won't start the season in Washington next year, then there's a risk to premature call up. Also, if you think the UCL is still dicey, then you probably don't want a Cavalli scenario in which he accrues service time on the 60 day IL for a year plus.