Alex Ferguson challenges Wayne Rooney

Sir Alex Ferguson has warned Wayne Rooney he must prove he is worthy of a place alongside Robin van Persie as the pair look to start their first Premier League match together for Manchester United.

Ferguson might have a central defensive injury crisis, but he is spoilt for choice when it comes to forward options and says he does not know "where I am going in terms of what my best partnership will be".

Both Javier Hernandez and Danny Welbeck impressed during Rooney's four-match absence caused by a bad gash to his right leg during United's 3-2 victory over Fulham at Old Trafford on Aug 25.

Mexican international Hernandez "has done nothing wrong" said Ferguson ahead of today's (Saturday's) game with Tottenham Hotspur, where victory would send United to the summit, if Chelsea failed to beat Arsenal, for the first time since they conceded top spot to Manchester City three games from the end of last season.

As for 21-year-old Welbeck, Ferguson added: "Danny is promising and he has shown that with England. He is in the same position as Chicharito.

"Being the kind of young lads they are, they are desperate to establish themselves in our first team. That is what competition is. We had it in 1999.

"It did a lot of good for Andy Cole and Dwight Yorke because they knew Teddy Sheringham and Ole Gunnar Solskjaer wanted to play and they did get a lot of football. I was able to change them around several times.

"It is early doors but in the situation we have at the moment I don't know where I am going in the sense of what my best partnership will be because I haven't seen Rooney and Van Persie together. I could play any combination."

Rooney has managed just 44 minutes alongside Van Persie since United invested pounds 22?million to secure the services of the Holland forward from Arsenal last month.

Neither performance, a 1-0 defeat at Everton nor the win over Fulham, was particularly sparkling. But Ferguson hopes Rooney will flourish alongside Van Persie now he has made a full recovery from injury after helping United overcome Newcastle 2-1 in the League Cup on his return on Wednesday.

The sight of Rooney and Van Persie starting together should at least help to cushion the blow of Nemanja Vidic's prolonged absence, Ferguson admitting yesterday that he had spotted the defender's latest knee injury coming after the 30-year-old Serb missed the final five months of last season with a ruptured cruciate knee ligament.

"I don't think he was ever 100 per cent," the United manager said. "I noticed it a couple of times in training that he was limping a little bit and then he started feeling tightness in his knee.

"Obviously the main operation was to correct the cruciate but when you do cruciates there is always the danger that the cartilage or meniscus is damaged also and that was the case.

"He wanted to carry on and in that situation that is what tough guys can do."

With Phil Jones and Chris Smalling also sidelined, United are seriously short of experienced central defensive options, but first-team coach Rene Meulensteen has denied training methods are at the root of the problem.

"I can put that to bed and reassure all Manchester United fans that we have got the best sort of medical, strength and conditioning, organisation, around the team," said Dutchman Meulensteen in an interview in the latest issue of United fanzine Red News.

"We don't leave anything to chance. The players are tested every morning, they will be looked at.

"If we just suspect something we take them out, we give them a different programme. You can say, 'Well, why are Paul Scholes and Ryan Giggs not injured, they are the oldest ones'.

"We manage them really, really well and yes, sometimes you have got that stroke of bad luck. The problem is if you look to most of the injuries, Vidic, impact injuries.

"Most of them have been some sort of impact injury and I always do find that that is what people need to realise. You will see, the players that are all going into the Euros, and the Olympics, you have to keep a really close look on them. They are the ones that are going to pick up things later on."