The incident allegedly occurred in the warm-up before the last-16 first leg tie but Ronaldo showed no ill effects during United's 1-1 draw in France.
"We reported the matter to Uefa before the game," said Manchester United manager Sir Alex Ferguson.
"We noticed it in the warm-up and they (Uefa) are aware of it."
Carlos Tevez's late goal ensured United head to Old Trafford on level terms after Karim Benzema had given the hosts the lead.
United were stunned after the break when the 20-year-old striker turned on the edge of the area and fired a rasping drive into Edwin van der Sar's left corner for his 24th goal of the season.
Late in the game however, Ronaldo had a vicious free-kick tipped over the bar and Tevez struck after Lyon made a hash of the resulting corner.