I keep reading this nonsense that you can't have three rookie defensemen (I guess the left off part is "and have a winning team"). Yet, I remember when the Flyers had their 35 game unbeaten streak, they had three rookie defensemen, albeit rookies with a ton of AHL experience, but far less talent that Sanheim, Hagg and Morin. This team is far better with Sanheim in the lineup than MacDonald or Manning. I would also argue that the team is better with Hagg than either of those two, but it is closer. Further, I would argue that Morin brings a much needed dimension that makes the team better than either Manning or MacDonald. To me that is a no brainer, keep the three kids and send one of MacD or Manning to the minors. If you think you are more likely to lose Manning than MacDonald due to their salary differences, you send MacDonald down and dare someone to take tat salary and let Manning ride the press box seat until needed. Any significant injury to a defenseman, or regression by one of the kids, you can easily bring MacDonalad back up. This isn't hard.
Are Manning and MacDonald NHL quality defensemen? Yes, but bottom pairing guys at best, and you can pair any bottom guy with a number one, and be alright, but not as good as pairing the number 1 with a quality guy. So I would send MacDonald down and keep the three kids and see what happens. It's not like you are really going to challenge for the Cup this year anyway.