Ipswich Town boss Paul Jewell has said he is mystified by his side's poor performances in their last two matches.
Town beat Portsmouth with an under-par showing on Tuesday, but lost 1-0 to Crystal Palace at the weekend.
Describing both displays as "flat", Jewell told BBC Radio Suffolk: "I don't know what's caused it, it's my job to find out.
"Training's good, preparation's good, the opposition have just played better than us on the last two occasions."
He added: "On one occasion we got away with it, I said after the game on Tuesday if we play like that we won't win."
Jewell opted to change his forward pairing following the victory over Pompey, with Michael Chopra and Jason Scotland making way for Jay-Emmanuel Thomas and Nathan Ellington.
Following a first defeat in seven games, Jewell defended his decision.
"Michael and Jason have been carrying little knocks and haven't been 100% fit," he said.
"With a third game in a week I decided to give Nathan and Jay a go. At times they caused them problems, but not for long enough periods - but that's not just down to those two."
A first-half foot injury to Damien Delaney saw Ivar Ingimarsson make his first appearance since the opening day of the season.
Irishman Delaney will be assessed before Saturday's trip to Millwall, but Ibrahima Sonko could make a timely return from a hamstring problem.
"Hamstrings are always difficult injuries, if you go out there and tear it you're out for a long time," Jewell continued.
"It was a niggle and hopefully he'll be in contention for next week. Damien hurt a bone in his foot first half, so we'll have a look at that."