A spokeswoman at Stack's Thomastown Castle base in Co Tipperary said: "The doctors are happy with his progress. There has been some improvement and his condition has stabilised."
Stack, 53, has held a training licence since 1986. The highlights of his career include the victory of Las Meninas in the 1,000 Guineas of 1994 and Tarascon's triumph in this year's Irish 1,000 Guineas.
Stack was twice champion National Hunt jockey but the high point of his riding career came when he steered Red Rum to the horse's record third Grand National win in 1977.
Cumbrian Challenge provided a shock for followers of the form book when winning the Castleford Chase at Wetherby yesterday at 16-1 from Direct Route. The nine-year-old's performances this season gave little cause for optimism and only two days ago he had been beaten more than 30 lengths.Reuse content