Pompey suffered their first home defeat of the season to fellow promotion candidates Doncaster Rovers thanks to a sluggish first half performance.
The damage was done early on and Rovers took the lead after just seven minutes. David Forde came rushing out of his goal to deal with a long ball but ex-Pompey loanee John Marquis beat him to it, took the ball past him and slotted it into an empty net.
More dreadful defending allowed the visitors to double their lead as a simple long free kick was headed on to Andy Williams who wasn't tracked by either Christian Burgess or Matt Clarke, enabling him to beat Forde with ease.
Going two goals down got Pompey into gear. Gareth Evans was booked for diving (it was a soft challenge, to be fair) but Carl Baker tested Donny keeper Marko Marosi with a 30-yard drive and then a Gary Roberts cross was almost turned into an own goal by Cedric Evina.
Pompey were then gifted an equaliser when Jordan Houghton needlessly intercepted a cross that was sailing out for a goal kick, the ball dropped to Baker and he lashed it into the roof of the net to reduce the deficit.
The second half was an open one. Conor Chaplin wasted a good chance by his standards to equalise, Burgess planted a free header wide of the post and Baker diverted Milan Lalkovic's shot onto the crossbar.
Rovers constantly threatened on the counter-attack but were denied by some last-ditch defending or Forde, who had to make a decent save when Clarke's mistake presented Doncaster with a one-on-one chance.
But when Curtis Main botched a great chance to equalise from close range and Roberts' header was hacked off the line, the Pompey fans just knew it wasn't going to be their day as six minutes of stoppage time failed to yield a goal.
This was Pompey's first big test of the season and they failed. If fingers of blame are to be pointed then Forde, Burgess and Clarke have to take the brunt of it.
It's worrying that someone who Pompey fans believed had solved the goalkeeping crisis has now made two critical errors in consecutive games. However, it should be said that Forde's saves during the game, when he was let down by the men in front of him, kept his team in it.
The disorganisation of the defence, especially when dealing with long balls, is a continual concern. Putting Clarke back in the team at the expense of Tom Davies didn't change anything and it's clear that Matt is still rusty after a long lay-off.
Cook can't really be blamed too much for the defeat as his tactical switches almost paid off, but his players let him down. The midfield duo of Doyle and Rose were all over the place with Amine Linganzi only a slight improvement over the latter whilst both Chaplin and Main missed their big opportunities.
We, as Portsmouth supporters, need to have some perspective as although Pompey lost their first "six-pointer" of the season, there will be many more to come in what is a long campaign. There's still plenty of time for Cook to get it right.
What we will discover now is just how much character he and his team have. Pompey didn't lose three matches in a row at all last season but that is now a very real prospect with a trip to Brisbane Road to take on bogey team Leyton Orient next week.
As always, supporting this club is going to be a rollercoaster.
Pompey Player Ratings
David Forde: 5 - At fault for the opener but made some good saves.
Gareth Evans: 5 - Starting to look a bit jaded at right back.
Christian Burgess: 5 - All at sea.
Matt Clarke: 5 - Needs match practice.
Enda Stevens: 5 - Has had better days.
Michael Doyle: 5 - Made some crucial blocks but also missed several key challenges.
Danny Rose: 4 - Unsurprisingly hauled off early.
Gary Roberts: 6 - Whipped in some good crosses.
Carl Baker: 7 - Would have scored more than one on another day.
Kal Naismith: 5 - Offered little.
Conor Chaplin: 5 - Struggled up top on his own.
Substitutes
Liam O'Brien
Tom Davies
Amine Linganzi (Rose 54): 6 - Steady job.
Milan Lalkovic (Naismith 54): 7 - Looked bright, should be starting more often.
Kyle Bennett
Michael Smith
Curtis Main (Chaplin 71): N/A - Wasted his one big chance.