Los Blancos host the Blaugrana on Wednesday in the second leg of the Copa del Rey semi-final with the score tied at 1-1, but then welcome Barca again in La Liga on Saturday.
Speaking to Ignasi Oliva Gispert of Goal ahead of the cup match, Milla said neither team are currently at their vintage best.
"I don't think there's a favourite, even if one side comes into it in better form than the other. The only substantial difference that I see between both teams is Messi, but they are generally games without favourites. The home sides might have a slight advantage because of their fans, but I don't see a clear favourite. Madrid are under a lot of pressure so you don't know what will happen.
"The good thing about Barcelona is Messi, he can unbalance anyone, and Gerard Pique and Marc-Andre ter Stegen are performing at a great level. Still, there's a feeling that both teams have to improve collectively if they want to win the Champions League – neither of them have a chance if they don't."
The first-leg away goal gives Real a slim advantage, but Barca have proved they can dismantle their Clasico opponent by force this season.
The Catalan giants beat Madrid 5-1 at the Camp Nou in October, and Real president Florentino Perez soon dismissed Julen Lopetegui as manager.
Los Blancos have consolidated under new coach Santiago Solari, but they have failed to discover the lightning form which helped them win the UEFA Champions League three seasons running.
However, despite their dominance in La Liga, Barca have uncharacteristically stumbled with two wins in their last six in all competitions.
Real are realistically out of the title picture in La Liga, but the Copa could give them a shot in the arm before the end of the campaign.
The team have failed to win the competition for five years, allowing the Blaugrana to grab four Copas in a row.
Halting that cycle won't bring huge satisfaction to Perez, but it could buy Solari extra time in the hot seat as he attempts to speed up his club's recovery after a miserable campaign.