Colchester United have signed American defender Will Packwood on loan from Birmingham until 1 January after Magnus Okuonghae ruptured an Achilles tendon in the 1-0 defeat by Coventry.
Club captain Okuonghae, 28, has made 11 appearances this season, scoring once, but also being sent off twice.
"It's a massive blow and I really feel for Magnus. He could miss the rest of the season," said manager Tony Humes.
"He's had a really stop-start season and now has this to contend with too."
Packwood has made 21 first-team appearances for Birmingham, but only one this season in a 3-1 win at Millwall in September.
The 21-year-old played eight games during a loan spell at Bristol Rovers last season.