Ronald Koeman has heaped pressure on rival manager Louis van Gaal by insisting Manchester United have to be challenging for the Barclays Premier League given the money he spent in the summer.

United are nine points behind leaders Chelsea and Manchester City, both on 46 points, and face Koeman's Southampton in a crucial match on Sunday.

Van Gaal was backed in the transfer market last summer to the tune of £150million – which included buying British record signing Angel di Maria for £59.6m – yet he has twice played down his side's chances of winning the title.

Ronald Koeman heaped pressure on rival Louis van Gaal by saying Manchester United need to be challenging for the Premier League title having spent a massive £150m on new players over the summer

Van Gaal's United welcome Southampton to Old Trafford on Sunday in a meeting of third and fourth

Just one point separates Manchester United and Southampton in the Premier League table

But Koeman, who has shared a fractious relationship with his counterpart since they fell out at Barcelona, claims Van Gaal has no excuses for not challenging for the title this campaign.

'They have great players so it's normal that they will fight for titles,' Koeman said. 'They have to. How you can spend that money, how can you sign that kind of players, and not be fighting to win titles?

'Everything is up to Manchester United. They have a very successful coach, they have great players, they have money, they have great fans, a great stadium. It's normal that you win titles.

'We had some problems in the past. We shake hands and I hope he will do well with United. It would be special to beat him, but I lost the last time and we try to put it right this time.'

Van Persie scores what proved to be Man United's winner in their 2-1 success at Southampton in December

The Dutchman scored two goals at St Mary's in an exhibition of clinical finishing

Old rivals Van Gaal and Koeman shake hands before the match at St Mary's on December 8

Southampton deserved at least a point against United when they lost 2-1 at St Mary's back in December, but two clinical goals from Robin van Persie decided the game.

The match at Old Trafford on Sunday could see United pull four points clear of Southampton or result in the south coast club leap-frogging their opponents into third.

Koeman added: 'I believe we belong in the top four. I hope the players show that as well. Our recent victory against Arsenal was important and the draw with Chelsea.