The Red Devils recorded successive Premier League victories at Vicarage Road thanks to Manchester United’s 2-1 win over Watford.
Manchester United appeared to be coasting following goals from Romelu Lukaku and Smalling in the English capital.
However, Watford halved the deficit in the second half through Andre Gray before Nemanja Matic was sent off for a second bookable offence.
The Hornets appeared set to equalise in the dying moments of the game but De Gea produced a world-class save to deny Christian Kabasele’s header.
Smalling lavished praise on De Gea for his latest key save in the Manchester United goal.
“Ultimately, for most of the game, he didn’t have much to do, but what makes him one of the best in the world is that he doesn’t have much to do and stands up in the last minute when it is a crucial time, and that’s where the best goalkeepers are,” Smalling told Man United’s website. “They don’t switch off and David’s always there!”
Manchester United will start their Champions League campaign with a home clash against Young Boys in Switzerland on Wednesday night.
The Red Devils have conceded eight times in five games in the Premier League this season following losses to Tottenham Hotspur and Brighton.
Manchester United haven’t won the Premier League title since 2013 when Sir Alex Ferguson was still in charge.
The Red Devils will be hoping to make it past the last 16 of the Champions League following a 2-1 loss to Sevilla at that stage last term.
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