Sergio Romero says Manchester United are a threat due to their tactical flexibility.

Ole Gunnar Solskjaer has changed between a back-three and back-four in recent weeks.

"They are two quite different formations really," Romero told the club's website.

"When you play with a four, you know that one of the two full-backs will get forward while the other tries to sit a little more.

"But I reckon that when we play with three at the back, our team is much more stable and solid. That's down to the fact that we are playing with three central defenders who are not going to get forward and join the attack too much, but who do have the necessary ability to be able get forward if needed.

"They can play with the ball at their feet and they can also play without the ball, by pressing a little higher up the field because they can easily push out when not in possession of the ball, in the knowledge that they have one extra central defender. It's not like a midfielder who pushes up the field to press the ball, but rather it's a central defender who is pressing higher, and a central defender is a lot stronger than a midfielder when he is pressing and closing down.

"As goalkeepers, we are used to all this and we know that we can just as easily play with a four as we can with a three at the back. There's no problem at all in that.

"Our role at the back is to always help out our defence by making sure everyone is in the right place, whether it be four or three or if it's a two-against-two. We are always going to be ready to help out our team-mates by shouting so that defending becomes easier."