Morgan Schneiderlin said that playing for Southampton in League One toughened him up and made him the player that Manchester United wanted to sign.
Schneiderlin, 25, joined United for £25 million from Southampton on Monday, capping a remarkable rise from the bottom of the third tier.
The midfielder had signed for Southampton as a 19-year-old from Strasbourg in 2008 and Saints were relegated from the Championship in his first season at St Mary's.
Southampton began the 2009-10 season in League One with minus 10 points, and Schneiderlin spent two seasons in that division before helping them win two promotions and finish seventh in the Premier League.
Schneiderlin was unveiled at a news conference in Seattle on Wednesday and said: "The best thing that happened to me when I came to England I was a young guy and every manager told me technically I was good but I had to toughen my play so the best way to learn that was in the second division in England.
"As you know it is fighting all the time so I could improve my game that way. Like I say, the manager made me improve and now I am very grateful for what I had.
"Today is the next step in my career, a new environment and very demanding but I am ready for that and to learn even more and to play."