Any time someone is new to the league, they can have great success because the league has not seen enough of him to find and exploit his weaknesses. If promoted too soon, it can put him in a position where he is destined to fail. I would much prefer to have his weaknesses exposed in a league where he is mostly their equal, rather than have him work on many weaknesses at once.
Give the league time to expose his flaws, and give him time to cure those flaws. If they don't find flaws after a few weeks, there is plenty of time to promote him.