I would absolutely love to have Stones at United, but not at this moment in time. The club still has a half-baked defense, and adding another young player wouldn't help alleviate some of the concerns regarding experience and genuine leadership ability at the back, something that could become more of an issue if De Gea leaves this summer or next. Smalling showed some encouraging grown in that department last season, but we can't risk putting all our chips on him. The club needs someone who can slot right in and assume control of the defense, which makes any potential bid for Stones highly unlikely, this season at the very least and maybe next season too.
Would be disappointed if he joins Chelsea nonetheless. He's probably the best defensive prospect England has produced in recent years. The thought of him honing his craft under Mourinho, and leaning on Terry's experience to form a partnership with Zouma, just ahead of Courtois is a bit unpleasant to say the least. Luckily for us, there are some bright central defensive talents around Europe in the form of Gimenez, Rugani, Laporte so even if we miss out on Stones, it wouldn't be the end of the world, apart from the homegrown detail.