Indeed, if the strength of the two matches is more towards a draw. However, I see that AC Milan is a little under pressure to achieve their ambition of getting 1st position back. Defeat against Spezia can make AC Milan players less confident in facing Inter. If, Milan players are able to forget the result of this match and have the confidence to beat Inter again after losing the previous derby, then this could be a good motivation for Milan players. However, Inter players have better confidence after beat Milan in Coppa Italia. This is what makes me a little doubtful that AC Milan can win.
Spezia game was really an interesting one for Milan. But on the contrary to your opinion, I think Milan will not be affected by the result of this game much. It was a weird game and I think Milan already forgot about it and started to focus on the Inter game. They will want to make up for that loss against Inter of course and they know that this is one of the important chances to take the lead again. I think Milan will defeat Inter again like they did in this season.