Manchester United star Romelu Lukaku was forced to catch up to the team bus in a taxi on Saturday after the Belgian was kept behind for a random drug test, according to The Sun.

Their summer signing scored the only goal of the game in United's trip to St Mary's Stadium, sending the Red Devils joint top of the Premier Lague table with their Manchester rivals.

After being selected at random to take part in the drug test, Lukaku took much longer than expected because he struggled to provide a urine sample, causing the Manchester United team bus to drive off without him.

José Mourinho was full of praise for his new striker following their victory against Southampton. Having previously worked with Lukaku at Chelsea, the Portuguese manager said that he wasn't expecting Lukaku to hit the ground running like he has.

"I knew he would always score goals, I knew he would always score more goals for us than for Everton or West Bromwich Albion. That’s logic," Mourinho said, as reported by the Mirror. "But I was not expecting him to be match after match, putting the ball in the net."

United are now preparing for a Champions League trip to face CSKA Moscow in Russia, before retunring to domestic action at home to Crystal Palace. The Red Devils then face their biggest test of the season in mid-October, making the journey to Anfield to face Jürgen Klopp's Liverpool.