Ronaldo is currently on international duty ahead of Portugal's clash with Sweden on Saturday night, but the images - published on the Daily Mail's website - suggest the forward could miss the 2010 World Cup qualifer and several crunch clashes for the Premier League leaders.
Injury to one of his prize assets will be the last thing Ferguson needs as the Red Devils bid to return to winning ways following successive league defeats to Liverpool and Fulham.
And with Paul Scholes, Nemanja Vidic and Wayne Rooney all suspended and Dimitar Berbatov struggling to be fit for United's next league clash against Aston Villa, the Portuguese's presence is essential.
Sunday's clash with Villa at Old Trafford will be the first of seven matches for United in the month of April, with Sir Alex's side also facing Sunderland, Portsmouth and Tottenham in the league, Everton in the FA Cup semi-finals at Wembley and Porto (twice) in the Champions League quarter-finals.
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