The Blues strolled to a 3-0 success over 10-man Morecambe on Saturday – a maiden win of their League Two campaign.
It was also the first time this season Whittingham’s suspect defence has managed to avoid conceding a goal.
Painter was thrown in at left-back following his signing on Thursday.
Pompey’s boss was attempting to shore up a defence which had conceded seven times in three matches.
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The result was a solid bow from the former Brighton & Hove Albion man as the goal remained unbreached at Fratton Park.
And Whittingham saluted the immediate impact of the newcomer.
He said: ‘Marcos was very steady.
‘I was pleased with his performance.
‘He was quite solid for us, filled in on that left-back spot very well and did what he had to do.
‘It was very pleasing to have him.
‘You would like to think he had an impact on that defence.
‘Although, you are not going to go on about it because football can knock you down straight away.
‘He had a good game, though.
‘I think his calmness, understanding and positioning was great.
‘Marcos also took a corner and had a shot from a well-worked corner routine.
‘We were never worried about his fitness going into the game.
‘He told us he had a couple of games anyway with Birmingham’s development group, so we weren’t worried at all.’