The goalkeeper, who was on the Old Trafford club’s books from 2005-2010, reflected on his cup success ahead of his side’s FA Cup semi-final clash with Wolves on Sunday.
Foster won three consecutive League Cups between 2008 and 2011, the first two with the Red Devils and the third with Birmingham City.
Looking back on the achievements, he insists United’s relentless quest for perfection meant that the trophy he lifted with the Blues was a more special occasion.
"I won the League Cup when I was with Birmingham and that was one of the highlights of my career," Foster told ESPN FC.
"We played Arsenal that day at Wembley and were the underdogs like you wouldn't believe. But we did win and, when you run down the pitch seeing all the fans going absolutely crazy, you see how much it meant to them.
"That will live with me for the rest of my life and to do something like that with the team at Watford, where it would mean exactly the same sort of thing, would be would be huge for me.
"I won the League Cup when I was with United, but it was almost a given that we were going to win the game.
"I remember getting back on the coach after the game and it was basically, 'Right, training tomorrow lads, we've got a Champions League match on the Tuesday'.
"It was all forgotten as soon as that. It was incredible. There was no celebrating, no kind of really revelling in it and that's the difference for me, you know?
"I just couldn't get my head around the fact that it was just another trophy. Do you know what I mean? But I think, if you win it with a team like Birmingham, West Brom or Watford, it does mean so much more."
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