Saturday's 1-0 loss at Sheffield United left Pompey three points from safety, and while it is still early in the season the club are bottom of the table.
"We are not fit enough yet. The only thing the players have done is played the games this week," said Cotterill.
"None of our boys have had a pre-season. We haven't had a good start that has given us a base fitness-wise. The games come thick and fast and we would have preferred a week's training."
Preston North End are also in the relegation zone, losing 1-0 at home to Norwich on Saturday.
"It was disappointing we have given our worst performance of the season on the day we have got our largest crowd here," said Darren Ferguson, the Preston boss.
"It was important to put a good performance on and get a win and hopefully get a few fans back, but it was a performance that was lacking in confidence, energy, tempo.
"As I said to the players we have got ourselves into this mess and a bit of trouble and no one is going to help us get out of it."
The standout result of the weekend was Watford's 6-1 win at Millwall, a result made all the more surprising given the Lions' impressive start to the season.
"You have to match Millwall's work-rate and nullify their strengths. We went at them and applied ourselves from minute one. Invariably we did that," said the Hornets manager Malky Mackay.
"It's down to team spirit - we are a young squad and maybe that helps us. We have a sense of togetherness. Everyone looks at anomalies but we deserved it.
"It can be very tough at the Den but we scored early to keep them quiet. We have belief and ability. We have no egos. I know how tough this division can be."