Manchester United legend Sir Alex Ferguson is unarguably the greatest Premier League manager of all time. He built many great teams at Manchester United during his 26-year tenure with the club, winning the league title a record 13 times.While his focus was primarily on the Manchester United academy and bringing out the best of talents, it isn't quite possible to build a world-beating side without major signings. Sir Alex brought in the best players from around the world to Manchester United, who helped the club win major honors.In this article, we will look at the five signings made by Sir Alex Ferguson at Manchester United who had the biggest impact:#5 Peter SchmeichelChamps League SF Peter SchmeichelOne of the greatest goalkeepers of all time, Peter Schmeichel joined Manchester United in 1991 from Denmark's Brondby for €750,000. Schmeichel's signing was later described as the "bargain of the century" by Sir Alex Ferguson:“The signing, when it took place in August of 1991, was for a fee of £505,000, which made it one of the bargains of the century in football. During most of his time with us, a golden spell in our history that was to finish perfectly with the European Cup triumph in Barcelona, Peter Schmeichel was the best goalkeeper in the world."In seven years with the Manchester-based club, Schmeichel won numerous accolades, including five Premier League titles, a Champions League, three FA Cups and a League Cup.UEFA Champions League@ChampionsLeagueGreat Dane ✅ 1999 winner ✅🥳 Happy birthday, Peter Schmeichel!@ManUtd | #HBD | #UCL3:11 AM · Nov 18, 20214400353Great Dane ✅ 1999 winner ✅🥳 Happy birthday, Peter Schmeichel!@ManUtd | #HBD | #UCL https://t.co/oGXV1wSFVtThe goalkeeper played an integral role in keeping the score down to 1-0 in the 1998-99 Champions League final against Bayern Munich. Ultimately, United scored two goals in injury time to complete a historic treble. Schmeichel left the club after the 1998-99 season and it took Manchester United six years to fully replace him when they signed Edwin van der Sar in 2005.#4 Wayne RooneyManchester United v Fenerbahce SKWayne Rooney joined Manchester United in 2004 as a teenage prodigy with the world at his feet and left the club in 2017 as their greatest goal-scorer. The €37 million that Manchester United paid for him was the highest transfer sum ever paid for a player below 20 years of age.Rooney made an immediate impact at the club, scoring a Champions League hat-trick on his debut. He remained an integral part of the United side that dominated the Premier League in 2000s.GOAL@goal16 years since Wayne Rooney’s debut hat-trick for Manchester United 🔴2:33 AM · Sep 28, 2020214432216 years since Wayne Rooney’s debut hat-trick for Manchester United 🔴https://t.co/SGNl4ZXJKAThe Englishman, more often than not, played as a second striker and was even shunted back into midfield during his time with the club. However, he never quite failed to deliver.In all, Rooney made 559 appearances for United, scoring 253 goals and assisting another 146. He won five Premier League titles, a Champions League, one FA Cup, four League Cups and was named Premier League player of the year for the 2009-10 season.