The 19-year-old has appeared just twice for Premier League side Saints, having made his debut in last season's FA Cup fourth-round defeat by Crystal Palace.
The former England Under-17 striker, who has yet to feature in the first team in 2015-16, is eligible to play in Saturday's match against Millwall.
His arrival follows Thursday's sale of striker Ryan Colclough to Wigan.
Crewe stand second bottom of League One, seven points adrift of safety.
Crewe boss Steve Davis is still hopeful of bringing back former striker Ryan Lowe for a third spell at Gresty Road, before Monday night's transfer deadline.
The much-travelled Bury veteran, 37, who is just four short of 200 career goals with seven separate Football League clubs, spent two seasons with the Alex under Dario Gradi from 2006 to 2008.
He then scored twice in six games when he returned to Crewe on loan on 23 November, during which time the Alex recorded two of this season's meagre haul of five League One victories.
"If we can still do the one we want to do then we would," said Crewe boss Steve Davis. "I have spoken to them and we need a decision because we may have to look at alternatives if it can't be done.
"If we can find another one who can do a job for us then that is a possibility as well. We won't go out and find a replacement for Ryan Colclough but one or two new players would give us a lift."
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League One strugglers Crewe have signed Southampton forward Ryan Seager on a one-month youth loan.