Manchester United striker Ole Gunnar Solskjaer believes the club's old guard will face real competition after manager Alex Ferguson's spending spree.
Ferguson has gone for youth by signing teenage Brazilian midfielder Anderson, 20-year-old Portugese winger Nani and 23-year-old Argentine striker Carlos Tevez, whose move from West Ham United should soon be completed.
England midfielder Owen Hargreaves has also joined as United prepare to defend their Premier League title.
Solskjaer told reporters on Tuesday: "The gaffer showed tremendous trust in the players we had last season. We hadn't won the league for three years and he only signed one player, Michael Carrick.
"That's a big sign of trust and he was right, we won the league.
"This season he's bought a few new young players in -- probably to challenge the older ones as well because you can't live off your past glory at this club."
The 34-year-old Norwegian, who is on his way back from a knee injury, believes the likes of Nani and Anderson will have to adjust to Premier League football.
But he pointed to Ferguson's track record in developing youngsters such as England striker Wayne Rooney and Portugal winger Cristiano Ronaldo, who scored 23 goals each last season.
"The young players are going to need time to settle in England, but the way we've nurtured Wayne and Cristiano, who came to the club as really young, talented players, the gaffer has made them into top world players.
"That's what you get at Manchester United, you get the challenge every day in training. They have to have that desire to improve all the time."
Solskjaer, who memorably scored United's stoppage time winner in the 1999 Champions League final against Bayern, was upbeat about returning from his latest injury.
"I'm smiling again," he said. "I'm training with the ball, running, twisting and turning, so hopefully it won't be too long before I'm out on the pitch."
Solskjaer spoke at the launch of a Premier League programme called Creating Chances, which uses funds from the league and its 20 clubs in community projects, charities and good causes.