Montgomery, 30 made his Millwall debut as a late substitute in a 3-0 victory at Doncaster Rovers on 20 March before picking up a calf stain in their 1-0 home loss to Neil Warnock's Leeds United; in all he played just 9 minutes for the New Den club and is unlikely to return to London once recovering from injury.
"It is a disappointment because he's picked up an injury very, very quickly at the start of his Millwall career," Lions boss Kenny Jackett told the Southwark News.