Sir Alex v Sir Arsene means goals. So might Guardiola v Hiddink

Both games could go either way.

Arsenal could win at Old Trafford if Rio and Vidic are not 100% fit and Fergie picks the wrong team.

And Chelsea could destroy Barcelona on set-pieces. If they get six free-kicks or corners, Chelsea could score four goals.

But the magical Messi could score four goals as well. What’s impressed me about Messi this season is his work off the ball, especially his runs in the box. We all know he’s the best dribbler since Maradona, and an electric improviser with superhuman balance, but Messi now scuttles between defenders and stabs in crosses from four yards.

Man United play Arsenal on Wednesday night and it’s a unique game.

They’ve never met in European competition. They’ve had replays but a replay is unlike a second leg, even if the first leg is  a 1-1 draw.This is Sir Alex v Sir Arsene over 180 minutes.

It was Harry Redknapp, not Howard Webb, who made the Manchester United- Spurs game ridiculous. Harry was 2-0 up at half-time and he sent them out to defend and slow the game down. How barmy was that? As soon as Harry did that, he’d lost.

The penalty, when Gomes dived the feet of Carrick and got the ball, was a shocking decision by Webb. But Spurs collapsed like a  house of cards.

At half-time I had phoned my brother Paddy.

“What will he do in the second half? Will he switch Rooney to the left.?” I asked
“That’s one of his options, he likes to do that.”

Contrary to what I wrote last week, Rooney was much better on the left than in the middle.

First half, Rooney was at centre forward, Berbatov behind him, Ronaldo on the right, Nani on the left.

United superstars were posing, preening themselves, with Ronaldo waving his arms around any time he was touched, sitting on the ground sulking for 30 seconds if he was tackled. United were ridiculously bad and Spurs hustled them, as all teams should do.

Tevez for Nani was the obvious switch, with Rooney on the left. 

The touchline restricts Rooney and helps him, anchors him, concentrates his energy, stops him roaming all over the field. If you give him the whole pitch, he’s all over the place.  

It was Rooney’s tasty pass into Carrick’s run that led to the penalty. Rooney scored two goals and got three assists. Spurs were 2-0 up, then 5-2 down. They conceded four goals in 14 minutes

In any semi-final Sir Alex does not commit too much too early.

He should go for a power play, driven by Anderson, right from the kick-off, but he will not do that. He will be cautious because he has been tactically humiliated so many times in the past. Therefore he prefers to control the game and win narrowly, as in Porto, where Ronaldo scored after 6 minutes with a 40-yard thunderbolt. For the next 84 minutes nothing much happened and United kept it tight and won 1-0.

At 7pm we find out if Wenger goes 4-5-1 with Song and we find out if Sir Alex is barmy. We’ll look at his team and see if he’s bungled it. He should play Tevez in the first leg, Berabatov in the second. But he’ll play Berbatov because he paid £32 million for him.  

At that point, I might have another bet. Last night I backed over 2.5 goals in the game. Both managers want it to be tight but both their defences are so dodgy that I think there will be goals.

After destroying Spurs, Rooney should play on the left against Arsenal. And Wenger won’t mind  that because Sagna is his best defender, and his most natural defender. Sagna is very quick and tenacious and he should have recovered from that virus by now

So it’s between Ronaldo and Berbatov for the centre forward role. As Arsenal have young Gibbs at left back, is that a reason to play Ronaldo on the right wing? Not really, because Ronaldo doesn’t want to beat Gibbs and cross the ball for Berbatov to score and grab the headlines.

Ronaldo is United’s main goal threat. The bigger the game, the more he wants be in the middle and shooting. After scoring with a 40-yarder in the quarter -final, Ronaldo thinks he can score from anywhere.

Barcelona keeper Valdez wobbled in a 2-2 draw at Valencia on Saturday. He needs a level of protection that Barcelona can’t give him, even with Marquez restored to the back line.

Having gone to the Stamford Bridge last week and seen Chelsea draw 0-0 against Everton, one thing stood out : Cech launches it 60 yards every time. He never throws the ball out.

Their brand of 4-3-3 is mostly diagonal football and flick-on football, and Malouda and Drogba are very adept at knocking a waist-high ball sideways with the side of their boot or heel.

Chelsea are very,very big and strong.  Barcelona don’t play such powerful teams in La Liga, even though Sevilla striker Luis Fabiano is formidably strong, fast and awkward,

When Lampard hits a long, high corner kick, Terry, Drogba, Alex or Ivanovic can score with a header. No other team in the world has such an aerial threat. Every time they concede a free-kick or a corner, Barcelona will be in trouble. Even if they switch Abidal to centreback.

Yes, of course Barcelona  are the most creative and delightful team in the world right now, and the most prolific goalscorers, and Eto’o might show Drogba he’s still the guv’nor. Will Messi and Eto’o show us that Alex and JT are carthorses? Will dynamic Dani Alves carry the game to Chelsea ? Will Ballack be a passenger or score the winner with a free-kick over the wall?

Will the crafty, bold Guus Hiddink outfox 37-year old rookie coach Pep Guardiola?

Barcelona v Chelsea  WILL be compelling entertainment. It could go either way. Same tomorrow.

I can’t wait for the biggest matches of the season to kick off.