Morrison, 26, attended the Blades' pre-season training camp in Portugal earlier this month.

He played in Manchester United's 2011 FA Youth Cup-winning side with Paul Pogba and Jesse Lingard.

"It's great to be back in England again and to get my career kick-started again," Morrison said.

"Before I went away [to Portugal], they made me feel very welcome.

"It was an easy decision to make - the Premier League, a great team and a great manager."

Morrison was rated as one of the best young players to come through the Manchester United academy during Sir Alex Ferguson's time as manager.

But his career has been hampered by various off-field problems and he made just three first-team appearances for United and 24 in four years with West Ham.

Lazio handed him a four-year contract in 2015 but loaned him out to QPR during the 2016-17 season and then in 2017-18 to Mexican side Atlas, where he scored four goals in 25 games.

He spent the early part of this year at Ostersunds FK in Sweden on a six-month deal.

"Ravel is an undoubted talent and we are delighted that we have put together a deal that suits all parties," said Blades boss Chris Wilder.

"We are getting a fantastic footballer with a tremendous pedigree and this is a great opportunity for Rav; we believe we can help him get back to the level he wants to be."