The 23-year-old left-back joined United from Dutch club Vitesse on a five-year contract earlier this week.
"Evra has been an absolute soldier," said Ferguson. "Nobody has played more games than him over the last five years.
"He has played with injuries. He has guts. He has been up and down that touchline for five years.
"But he's 31 and we have to protect the future."
Ferguson is confident that Buttner will prove to be a successful signing at Old Trafford.
"We've been monitoring him for a while," said the Scot. "He's a straight forward attacking, quick full-back, he will be a terrific addition to us.
"Evra for the last five years has played an average of 48 games a season. It's an incredible statistic and Alex comes now to challenge that position.
"Patrice is in his 30s and, although he is a fit lad, it gives us a good challenge in that position."
United will host Fulham in their second Premier League game of the season on Saturday.