The Oxford manager believes that will be the case based on the calibre of clubs who will be vying for promotion during the 2022-23 campaign.
And among that group will be Pompey, who he believes will be having a go as they look to avoid a seventh-straight season in the third tier.
Like the Blues, Oxford missed out on promotion and the play-offs as they finished eighth on 76 points.
That was seven points off sixth-placed Wycombe and 16 less than champions Wigan, who accumulated 92 points over the course of last term.
Danny Cowley’s side, meanwhile, claimed 10th spot with 73 points from their 46 games played as they fell well short of their ambitions.
Both teams have much to do over the summer to get back in among those much-coveted top-six places.
That task will be made harder, though, with play-off semi-finalists Sheffield Wednesday and MK Dons, plus beaten Wembley finalists Wycombe, remaining in the division.
The Chairboys lost 2-0 to Sunderland on Saturday to be denied a straight return to the Championship.
With Derby, Peterborough and Barnsley also dropping down from the second tier, Robinson believes a harder division lies in wait.
Robinson told the Oxford Mail: ‘The league’s going to be better.
‘Ipswich, Charlton, Portsmouth and Bolton will have a go.
‘Three of that four (who reached the play-offs) are still going to be in this league with Derby County, Barnsley and Peterborough coming down (from the Championship).
‘I only foresee it getting harder to try to stay in the top ten and that’s something we’ve got to be better at as well.’