The recently crowned Ballon d'Or winner was given his marching orders 15 minutes from time, with the score already at 1-1, for appearing to hit out at Gurpegi after having a penalty appeal turned down.
The away draw leaves Real three points behind La Liga leaders Atletico Madrid, who moved to the top of the table with a 4-0 win over Real Sociedad earlier on Sunday.
Ronaldo is now likely to be banned for another crucial league encounter against Villarreal next weekend and Ancelotti wasn't convinced his star man deserved to see red.
"I think it is difficult for me to judge because I wasn't there, but the images show that it was an exaggerated reaction," said the Italian.
Brazilian defender Marcelo was stronger in his criticism of the decision.
"It is hard when you lose a player in an unjust manner. It is obvious that the decision wasn't fair and that made it very difficult, although our intention is to win every game," he told Spanish TV station Canal Plus.
"Athletic are a very good team, but we had bad luck today."
