Note that turnaround came with a change of manager ... Gattuso now managing them. Supposedly a bit more defensive & solid (no surprise there then) but still lacking flair up front. I think they'll see off Arsenal.
And that was the main surprise for me, because I didn't expect Gattuso to be able to do anything there. Thought of him as just a new "retired player trying his luck with football management" but seems he turned the tides for them very well.
Good luck for them against Arsenal. As I said will always have a soft spot for Milan as long as they stay away from United.