Ex-Manchester United striker Michael Owen has claimed that his former club will struggle to make any significant signings in the summer transfer window.

Old Trafford boss David Moyes, who is expected to be given a sizeable budget by the club owners come the end of the season, has been linked with several high-profile players, including Bayern Munich's Toni Kroos, Porto defender Eliaquim Mangala and William Carvalho from Sporting Lisbon.

But Owen, who also played for United's rivals Liverpool, reportedly believes that the key players who could improve the squad have already been signed by Europe's elite outfits.

"I can imagine David Moyes wants to make a lot of changes in terms of playing staff, when you're Manchester United manager everyone thinks it's very easy to bring in players," Owen is quoted as saying by the Daily Star.

"But I look around and think 'who can they buy if you want to make a lot of changes?', the best players in the world aren't available, they're either at Barcelona, Real Madrid, Chelsea, City or Bayern Munich.

"Why would anyone want to sell any of those players? I'm sure David Moyes wants to buy, but whether he can get the right people for the right money and enough of them will be his biggest challenge."

Man United spent a club record £37.1m to bring Juan Mata to Old Trafford from Chelsea in January.