The Blues hold a seven-point lead on the domestic front, albeit having played more games than the rest of the top four, and will head into the second leg of their Champions League last-16 tie against Galatasaray at 1-1 following a draw in Turkey.
But Mourinho told the press: "Can we go all the way in the Champions League and the Premier League? I think it's very difficult. Our squad is quite short; two of our players [January signings Nemanja Matic and Mohamed Salah] can't play in the Champions League. Twenty players minus two players is 18, minus [the injured] Marco van Ginkel is 17.
"When I say someone, I mean the best teams in the world, so I think both are very difficult but let's go match by match."
Mourinho, who will be without David Luiz and Ashley Cole at Villa Park, also described the table as "fake" — with City capable of catching Chelsea by winning its three games in hand.
"We don't look at the table because the table is fake," he said. "If I watch Tottenham against Arsenal and want Tottenham to win, does that help? No.
"I think the team is motivated and strong but not because of the table. They know the table is fake. We have clear objectives for the next season but if we can do it this season we don't say no. We are very calm, very relaxed and that's important."