Roberto Di Matteo has suggested to Aston Villa's official website that a lack of conditioning was the reason why striker Ross McCormack was withdrawn at half time in Wednesay night's 3-1 surprise defeat at Luton Town in the League Cup.
Aston Villa bowed out of the competition at the first hurdle, despite taking the lead through Jordan Ayew on 13 minutes.
Jake Gray restored parity for hosts Luton on 35 minutes, before a second-half capitulation hindered Villa as Cameron McGeehan and an own goal from Jores Okore saw the Championship outfit eliminated early on.
The Italian manager raised eyebrows after the break by removing £11 million striker McCormack for Rudy Gestede which did not prove an inspired decision by Di Matteo.
It was only the 29-year-old's second match for Aston Villa since joining from Fuham last week, however, and the former Chelsea boss says his marquee signing - who played 77 minutes of Sunday's 1-0 defeat at Sheffield Wednesday - is not up to speed just yet.
"With Ross, he’s only had three or four training sessions with us. We need to get his conditioning up as well. Those were the changes," he told Aston Villa's website.
The Villans, still hunting a first win under Di Matteo, host Rotherham United at Villa Park on Saturday.