Jose Enrique believes Manchester United face a "very difficult" challenge to beat Atletico Madrid after the two clubs were drawn against each other in the Champions League last-16 stage.
The former Liverpool defender, however, has pointed to Cristiano Ronaldo as the one player who could change the game in an instant.
Diego Simeone's side has already faced Premier League opposition in Europe after being drawn in the group stages with Enrique's former employers.
In their two meetings in Group B, Jurgen Klopp got the better of his opposite number both times with Atletico going down to 10-men during both clashes.
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