UEFA on Monday opened an investigation into Cristiano Ronaldo's goal celebration after scoring a hat-trick in Juventus' Champions League comeback win against Atletico Madrid last week. European football's governing body said Ronaldo was facing charges of "improper conduct" after he mimicked Diego Simeone's crotch-grabbing 'cojones' gesture in last Tuesday's 3-0 win that sent Juventus through 3-2 on aggregate.