How to handle supervisor for intern

I'm not sure what words to use to explain an unsual supervisory situation to my new team on Wednesday.  The only reason it's awkward is because I am not at liberty to share many of the relevant details with the team, so I'm trying to figure out how to present it at the staff meeting.  

I have a senior financial analyst on my team the Controller has suggested be given supervisory responsibility for an intern that starts on Tuesday after Memorial Day.  The senior analyst will be promoted upon successful completion of a special project that is slated to be finished in June.  The rest of the team is unaware of this special project or the senior analyst's impending promotion.  There are 3 other senior analysts on my team and 2 analysts.  

The reason the Controller made this request is because 1) I was promoted from a principal analyst on the team to manager of the team effective today, so I have 6 new direct reports and 1 open position + the intern starting Tuesday and 2) the Controller and Director are both aware of the senior analyst's impending promotion and feel confident she can successfully supervise the intern.  

I can't say the best thing, which would be "XXX has achieved X, Y and Z in 2014 and completed a special project.  Based on the results of those project,s she will be promoted to Principal Financial Analyst on June 1.  Since I will be out of the office when the intern starts work, the intern will report to XXX in her new role."  It's more likely the promotion won't be effective until July 1 and I can't share that information.  

Do you think something like "Since I now have 6 new direct reports (all of you) and will be on vacation when the intern starts, the intern will report to XXX." would work?   I feel like this explanation might generate resentment because two of the seniors were hired near the same time as XXX and it seems to lack an apparent reason.  I could mention her achievements and perhaps that would help?  I don't want to pass the buck and say "The Director recommended..."

Other facts that may be relevant to your view of the situation:

• I will be out of the office on vacation the week the intern starts and have never met him.  Other than what's on his resume, all I know about him is that he's "very tall" (he's on his college basketball team).  Since I haven't met him, I can't make any guess as to whether he will mesh well with the senior analyst.  I talked to HR, but there's nothing I can do to move his start date.  This, plus all of the new direct reports are both good reasons for him to not report to me.
• Our team manages accounting, actuarial, investment and finance projects with cross functional teams, performs technical accounting research on a huge array of topics, coordinates the Company's relationship with its auditors, prepares and submits certain financial-based compliance documents to various State insurance departments, coordinates exam requests from state insurance departments and reviews them, reviews all financial statements prepared by the Company (I'd guess roughly 30 different sets prepared under various accounting bases) for technical accuracy, and manages the implementation of new accounting standards. 
• The senior analyst in question is very technically skilled, produces a lot of work, is organized, and manages projects very effectively.  However, she sometimes gets frustrated when she feels like people are going around her or not communicating with her appropriately.  A few of these displays of frustration have caused minor damage to her relationships with others on the team or at least alarm.
• I have been a principal on the team for 6 years and in that role, both directed work of many of the other team members and reviewed the work of those team members as a peer review.
• The manager position was open for several months.  The Controller wasn't originally sure I was the "perfect choice" for the position when I expressed my interest in it, which she considers "the hardest job in the department".  However, I got the impression she also wasn't expecting me to be interested in it.  I'm not sure she had thought about it, so she changed her mind about that just over a week ago.
• The team seems thrilled that I have been promoted.  I already have good relationships with all of the people on the team.