Delph picked up the problem ahead of Villa's 2-1 win at Portsmouth on Sunday and the club confirmed it was a 'serious' injury.
The Premier League side were awaiting the results of scans on the injury and they have now revealed the 19-year-old will be sidelined for the rest of the year.
A club statement read: "Fabian Delph suffered a rupture of the anterior cruciate ligament [ACL] in his left knee in training on Saturday at Bodymoor Heath.
"This has been confirmed by clinical examinations and scans.
"Fabian will undergo ACL reconstruction at the appropriate time in the near future and it is expected that he will be sidelined for a period of up to eight months."
The England Under-21 international joined Villa from Leeds United last summer, but he has yet to make a sustained impact in Martin O'Neill's first team.
He has made 15 appearances in all competitions for Villa, but has started only four games in the Premier League - the last of which was at Bolton Wanderers earlier this month.
Delph's only goal for the club to date arrived in the FA Cup fourth-round win over Brighton & Hove Albion in January.