Watch: Rookie goalkeeper's howler gifts Manchester United win at Benfica

Mile Svilar, the youngest keeper to play in the Champions League, was undone by Marcus Rashford's long-range free kick in the 64th minute

Benfica goalkeeper Mile Svilar carries the ball over the line as Manchester United's Marcus Rashford (not pictured) scored the only goal. Carl Recine / Reuters

An embarrassing mistake by Benfica's 18-year-old goalkeeper Mile Svilar gifted Manchester United a 1-0 win at Benfica in Uefa Champions League Group A on Wednesday.

The 18-year old Belgian, the youngest keeper to play in the Champions League, was undone by Marcus Rashford's long-range free kick in the 64th minute, catching the ball before back-pedalling and carrying it behind the goalline with his arms outstretched.

After breaking into tears at the final whistle, the son of former Serbian goalkeeper Ratko Svilar was consoled by United striker Romelu Lukaku, whose quiet evening underlined the English club's toothless performance.

The display was almost a carbon copy of Saturday's 0-0 Premier League draw at Liverpool but it extended their unbeaten run in both competitions this season under manager Jose Mourinho and left them top of the section with a maximum nine points.

Swiss side Basel are second with six points after a 2-0 win at CSKA Moscow, three points ahead of the Russian team, while Benfica are bottom with none.

Manchester United midfielder Nemanja Matic also expressed sympathy for Svilar but revealed Mourinho had instructed his team to capitalise on the keeper's lack of experience.

"The manager said to us to try and shoot and get some crosses towards their goalkeeper because he is young and playing in his first game," Matic told BT Sport.

"This is football and I know he is a great goalkeeper and I wish him well for the future.

"We had control, some difficulties in the first 30 minutes but then we controlled the pitch. We could not find the second goal but did not make any mistakes at the back."

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Mourinho, who had a short stint at Benfica as an assistant and then manager in 2000 in the early stages of his career, fielded an adventurous formation but the home side looked more likely to score in the opening half-hour.

Looking disjointed and bereft of ideas with a two-man midfield of Matic and Ander Herrera, the visitors carved out their first chance in the 36th minute when Svilar did well to block Matic's angled shot from 10 metres.

He also denied substitute Anthony Martial in the closing stages but all his good work was overshadowed by the howler which left Benfica's hopes of reaching the knockout stages hanging by a thread.

Benfica were reduced to 10 men in stoppage time when defender Luisao was sent off after receiving his second yellow card.