Ulster's hopes of securing the services of Stephen Donald until the end of the season have been scuppered after the fly-half sustained an injury.
Ulster had agreed a deal to sign the New Zealand World Cup winner to replace Christian Lealiifano who has returned to Australia to play for the Brumbies.
Donald was injured while playing for Tokyo-based Toshiba Brave Lupus in the final game of the Japanese season.
Ulster say it will be "very difficult to find a suitable replacement" now.
"Unfortunately, Ulster Rugby is not in a position to proceed with the signing of Stephen Donald after receiving a medical report on the player on Sunday," read a statement released by the Irish province on Monday.
"Stephen had agreed a short-term deal to join the province but he sustained an injury while representing his club in the final game of the Japanese season, which will rule him out for four-plus weeks.
"The timeline now makes it very difficult to find a suitable replacement, but we will continue to monitor the market."
The 34-year-old kicked the winning points in the 2011 World Cup.
Donald was on a fishing holiday when he was called into the All Blacks' squad and injuries to Dan Carter and Colin Slade saw him come off the bench in the final against France to kick the decisive penalty.
Lealiifano joined Ulster on a short-term contract in August but returned home after playing in the Champions Cup pool games against La Rochelle and Wasps.
Johnny McPhillips, Peter Nelson and Brett Herron represent Ulster's options for the number 10 shirt should they fail to find a replacement for Lealiifano.