Foster earned his first England cap in the defeat to Spain last month and is competing with Paul Robinson as boss Steve McClaren's first choice.

While Robinson missed Tottenham's quarter-final against Chelsea with a thigh complaint, Foster pulled off a string of saves later in the day at Home Park to help Boothroyd's men reach the last four.

"Ben Foster underlined why I think he will be the world's number-one goalkeeper," said Boothroyd, who has loaned Foster from Manchester United.

"He's got a terrific work-ethic but, more importantly, he has humility - he takes time to talk to people and sign kids' autographs."

Foster will not be allowed to play against United if they are drawn against Watford in the next round - "not unless we get surgery for him and pretend he is someone else," joked Boothroyd - and he has been backed as a long-term successor to Edwin van der Sar at Old Trafford.

"That's why Alex [Sir Alex Ferguson] wants him back, that's why he's got his cap and why he will go on and prove his worth," Boothroyd added.

"It's not my place to tell a knight of the realm or the England manager how to run their business, nor would I dare."

However, Boothroyd believes Foster is in good enough form if needed in the European Championship qualifiers later this month.

"If Ben Foster is called upon, he'll be absolutely outstanding. It's up to Steve to pick him," the Hornets boss added.

Foster was called upon to preserve Watford's lead after Hameur Bouazza had opened the scoring in the 22nd minute.

"He's ruined my dream with some great saves," said Plymouth boss Ian Holloway after the 1-0 defeat.

"He's cracked my heart."

Although his saves denied the Coca-Cola Championship side at least a replay, Foster was booked in the second half for time-wasting.

"If that's what gamesmanship is all about and winning games is all about, I've got to do more of it," added Holloway.

Holloway took no consolation from his side's efforts against Barclays Premiership opponents, although he added: "Can they give me much more? Yes, I think we can get in the play-offs. If we do that to Watford, why can't we do that week-in week-out?

"We threw everything at it - kitchen sink, golf clubs. We emptied the garage and threw it at them but it wasn't enough. At least my garage is tidy."

The victory puts Watford within sight of Wembley, which could earn a European place depending on this year's finalists.

"I'm thrilled," added Boothroyd. "We've taken knocks this year and haven't had luck in the league but we've had it in the FA Cup, I'm delighted to be in the semi-finals.

"It keeps it bubbling and keeps the momentum. It's a credit to the players that they kept going, we've got to because you never know.

"Everybody will want us in the draw. That's fine and perhaps we can use that."

Despite Holloway describing himself as a "failure" in defeat, Boothroyd praised his counterpart for his efforts in turning around the Devon club's fortunes.

"You can see Ian Holloway has this area buzzing, they'll use this to go further," he said.