Luka Modric
Luka Modric showcased his skills with some typically sublime passing as Croatia fell to a cruel defeat against Spain to exit Euro 2012. Reuters

Having been dethroned by their noisy neighbors in England and humiliatingly knocked-out of the Champions League at the group stage, there is work to be done this summer for Sir Alex Ferguson to get a final flourish of trophies before finally hanging up his hairdryer.

That work has already begun with the signings of Shinji Kagawa and 18-year-old Nick Powell. Kagawa, signed from Borussia Dortmund for an estimated £17 million, is expected to add vital creativity linking up with Wayne Rooney in and around the penalty area. The Japanese international has also proved to be a reliable supplier of goals in the Bundesliga and should help take some of the pressure off Rooney.

There remain, though, several areas of the squad, if United are to be able to keep pace with mega-rich Manchester City at home and the likes of Barcelona and Real Madrid further afield.

Here is a look at six players who would improve the United squad, along with videos of the stars in action. Of course finances and squad continuity dictate that not all these players can arrive this summer, but each could make a significant contribution at Old Trafford.

Luka Modric

The Tottenham playmaker has been tracked by Ferguson for some time. After appearing set for Chelsea last summer, United now appear to be front runners for his signature following Tottenham's failure once more to qualify for the Champions League.

Modric showed during Euro 2012 just why he is so highly rated as his precise passing and invention came close to leading Croatia to a surprise quarter-final place. His skills would be welcome at United where Paul Scholes' second retirement cannot be long away and Michael Carrick has failed to impose himself on the truly big occasion. Modric would certainly be an upgrade.

James Rodriguez

Ferguson loves a winger and James Rodriguez just might be the best young wide man around at the moment. As well as being blessed with ample pace and possessing tricks aplenty, the 20-year-old also has an eye for goal, having scored 13 in the league for Porto last season. No surprise, then, that the Colombian has been likened to former Manchester United great Cristiano Ronaldo.

Even more attractive to United will be that he would provide a complimentary left-footed option to the club's right-footed wingers--Nani, Antonio Valencia and Ashley Young.

Robert Lewandowski

Having already signed his teammate Kagawa, United has also been linked with a move for Borussia Dortmund striker Robert Lewandowski. The 23-year-old had a breakthrough season last campaign with 22 goals as Dortmund retained the Bundesliga crown.

With Dimitar Berbatov looking all but certain to be following Michael Owen out of United, there is a vacancy up front. While Javier Hernandez provides lethal finishing and a pacey threat in behind and Danny Welbeck also offers speed and improving link-up play, Lewandowski would provide the complimentary skills of a traditional number nine.

Mats Hummels

Yet another star of Dortmund's exceptional young side, Mats Hummels is currently impressing at the heart of Germany's defense in Euro 2012. Ferguson is apparently keen on the 23-year-old but it would require a substantial offer to pry him away from the Westfalenstadion this summer.

United has plenty of options at center-back, but while Rio Ferdinand's age and injuries mean there are concerns over his long-term future, neither of the three young defenders--Phil Jones, Chris Smalling and Johnny Evans--have yet convinced that they can be dominant figures against the very best.

They may yet prove to be so, but Hummels would certainly offer an immediate upgrade and, as these clips show, he's not bad on the ball, either.

Mathieu Debuchy

Reports claim that Newcastle is honing in on the right-back, who is valued at £6 million by his club, Lille. That represents a bargain for a player long seen as one of the emerging forces in his position in Europe. The 26-year-old has only enhanced his reputation with some fine performances for France at Euro 2012.

With Rafael enjoying an unconvincing campaign last season and Jones and Smalling both being used as makeshift right-backs, the club has yet to truly replace long-time servant Gary Neville. Debuchy would offer an forward thrusting ability in keeping with United's style.

Javi Martinez

Although he played much of last season as a center-back, Javi Martinez could be the holding midfielder Manchester United has been lacking for several season. Far from merely a destroyer, the Spanish international is also composed on the ball and would suit the possession style that Ferguson aims to deploy.

Admittedly, with Barcelona also said to be interested and Athletic Bilbao under little pressure to sell, the chances of the 23-year-old turning up at Old Trafford this summer appear slim.