The 18-year-old midfielder joins Saints from the English giants until the end of the season.
Named Manchester United Youth Scholar of the Year in May 2025, player captains the Red Devils' U21 side where he has made 22 appearances this season.
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Devaney has also been capped six times by Republic of Ireland at U21 level.
Speaking to the official club website , he said: "It’s a great opportunity for me to come here, do well and try to help the lads finish the season strong.
"As soon as the offer came up it was something that me and my family looked at, and thought was a great opportunity.
" I see it as a big step in my career to go and prove myself. Everything he [the manager] said was really positive and that’s why I’m here today.
"I’m a hard-working midfielder. I cover a lot of distance, I do a bit at both ends defending and like to get forward as well.
"Something I’ve worked on this season is my finishing, scoring goals and getting assists and I’ve added that this year."
Manager Stephen Robinson added: "Jacob gives us something that we haven't got.
"We haven't really replaced Caolan Boyd-Munce and with Keanu [Baccus] being injured we haven't had that link between defence and midfield in recent weeks.
"He's a really highly thought of young player at Manchester United so we're looking forward to getting him involved.
He's been playing U21 football for Manchester United on a regular basis and with our injuries he could get thrown in at the deep end, but we believe he's got a mentality to handle that and is someone we feel can make an impact."