Grimshaw has joined Well on loan until January.
He told the club's website: “I'm looking forward to it and it's a new experience for me as I've only ever played at Manchester Utd my whole career.
“First of all I'll be looking to get into the side, then I'll be looking to play as many games as possible and try to improve as a player.
“I've played a few positions for Man Utd, starting at centre back then right back but I prefer midfield. I like to get my foot in and pass the ball simply.
“I've watched a few games in Scotland and know it's a good standard and can't wait to get playing."
'Well boss Ian Baraclough added: “We are delighted to get Liam in and we hope he will add further strength to our midfield.
“He is a player we have watched regularly and have been patient trying to get him confirmed. Although he is young, he has been schooled at an excellent club and he'll add some steel in the middle of the park."