Highly-rated Atletico Madrid midfielder Saul Niguez has signed a new five-year contract.
Huddersfield will offer new contracts to Ishmael Miller and Oscar Gobern.
With Nick Chadwick one match into a six-game ban and Warren Feeney set to be sidelined for six-to-eight weeks with an ankle injury, Argyle boss John Sheridan has moved quickly to fill the gap with the former Yeovil loanee.
The 19-year-old will stay at Home Park for an initial month but the loan period can be extended until the end of the season, subject to an agreement between the Royals teenager and the two clubs.
Ugwu made five starts and 13 substitute appearances for the Glovers during his loan spell earlier this season, scoring three goals.