The boss feels the Blues’ upcoming five games in 15 days could be somewhat season defining in the chase for the Championship.
Saturday’s 1-1 draw at Plymouth means it’s now five games without a league victory for Pompey.
They’ve collected just two points from 15 and dropped from the top of the table to third place.
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To make matters worse, leaders Luton and second-placed Barnsley both delivered wins at the weekend, while Sunderland will leapfrog the Blues if they beat Blackpool tomorrow night.
Despite being held by Argyle, Jackett saw signs of improvement from his troops.
The Blues next travel Southend on Saturday before facing Bristol Rovers (February 19), Barnsley (February 23) and Bradford (March 2).
There’s also the Checkatrade Trophy semi-final at Bury on February 26 to negotiate.
And Jackett reckons that run could play a big factor in shaping Pompey’s ambitions.
The boss said: ‘I thought overall it was a good performance from ourselves.
‘We showed a lot, had good threat and could have scored more goals.
‘It was the best we played for a while but in isolation it was a difficult game.
‘We have a key period coming up now of five games in 15 days – four league games and the semi-final of the Checkatrade Trophy.
‘They’re great games and will, to some degree, define our season and if we can put in performances like we did against Plymouth then we won’t go far wrong.
‘It was always going to be a difficult game and a key one.
‘We acquitted ourselves well. I was pleased with Bryn Morris and Omar Bogle, two of the new lads coming in.
‘Saturday was the best we have played for a while and we are frustrated not to have won the game having went 1-0 up.
‘But Plymouth at home against ourselves is a big game for them. They’d won five out of six and were in a position were they could go for it.
‘It was a good performance anyway and I would suggest the best we have played for a while. We added a lot to our game and created chances.’
After a cagey first half, Ben Close opened the scoring for Pompey in the 56th minute.
Plymouth equalised through Graham Carey’s free-kick with 20 minutes remaining.
That set-piece was awarded after Nathan Thompson was adjudged to have brought down Yann Songo’o.
The Blues protested to referee Ben Toner it wasn’t a foul, though.
Jackett also felt that it was a harsh decision.
‘It did seem harsh at the time,’ the Pompey manager added.
‘It doesn’t really matter, though, and to be fair to Carey it was a fantastic strike.’
Omar Bogle had a big chance to double the Blues’ advantage in the 65th minute when he was one-on-one with Kyle Letheren but couldn’t get a shot away.
The on-loan Cardiff striker limped out shortly after with cramp, replaced by Oli Hawkins.
Jackett allayed fears over Bogle’s injury and feels it’s nothing serious.
‘He had cramp in his calves and it had been an effective performance from him,’ he said.
‘He got through one-on-one and the keeper smothered it and didn’t have that extra yard.
‘He felt a bit of cramp in both calves because he’d ran hard in the game.
‘We certainly hope that it is nothing serious.’