The former Blackburn man has been the Blues boss’ preferred partner alongside Sean Raggett during pre-season.
Paired with Sean Raggett, it represented just their third Pompey outing as a central-defensive double act.
Yet Jackett was keen to see them matched up – prompting Downing to be dropped from the 18-man squad.
He told The News: ‘Paul was not selected.
‘It was a bit tough on him, but I wanted to have a look at Jack Whatmough and Sean Raggett and then didn’t feel I needed another defensive player on the bench.
‘I had James Bolton, who can cover both right-back and right centre-half.
‘For Paul, it was one where he was a little bit unlucky, but similarly the main reason he wasn’t in last night was I wanted to look at Whatmough and Raggett.
‘They played well together, so we’ll see if they continue as the season goes on.’
Ahead of his Colchester run out, Whatmough had featured for just 180 minutes in pre-season, mainly in second-string sides.
He started at AFC Totton, while appeared as a half-time substitute against Norwich under-23s and Gosport.
However, he was unused in the behind-closed-doors defeat at Brighton last month.
Whatmough and Raggett had previously featured alongside each other against Scunthorpe in the Leasing.com Trophy in January and March’s League One outing at Peterborough.
As for Downing, his last league appearance was almost a year ago – in the September 17, 2019 match against Burton.
Pompey face Shrewsbury at Fratton Park on Saturday as their League One campaign gets underway.
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