I believe the red card incured by AC Milan affected them greatly. AC Milan is not an underated team and besides is coming from one of the top leagues Italy Serie A positioned in no 4 on the table without loosing a match yet. On the other hand FC Buonamount is positioned in no 15 in the English premiership winning just one match . This is to tell you that AC Milan would have been hard to defeat if they were complete to the end of this very match. However, it was very unfortunate for AC Milan to incure this red card that gave Buonamount to gain victory over them. Unfortunately to me I lost this prediction because I expected AC Milan to do better not considering any unforseen circumstances.
Even if Maignan hadn't received a red card, beating Bournemouth still wouldn't have been easy, and I think you know the difference between serie A and the EPL. So even though Bournemouth are 15th in the EPL, they're already used to facing top teams with a much higher level of performance. We can see that from the very first minute, Bournemouth kept putting Milan under pressure, and Milan's defense couldn't handle it. In the end, Maignan lost his emotional control.
So even if Milan had played with their full squad, the best result they could have hoped for was a draw.