The ex-Fratton favourite can't see his former side being stopped in League One this season.Â
The Blues remain at the summit of the table following their 2-0 victory over Walsall last night.
Kenny Jackett's men sit four points clear of second-placed Sunderland.
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What's more, there is a six-point gap separating Pompey and Luton, who are third.
Roberts, who won the League Two title at Fratton Park in 2017, feels his ex-club will remain in the automatic promotion places between now and the end of the season.
The Wigan midfielder believes the Blues and Sunderland will be plying their trade in the Championship next term.
Roberts wrote on Twitter: '˜Pompey flying ... can't see no one stopping them or Sunderland.'
The Liverpudlian moved to Fratton Park from Chesterfield in June 2015 after he was signed by Paul Cook.
He made 80 appearances in total, scoring 20 goals, and helped the Blues clinch the League Two crown in May 2017.Â
Roberts departed Pompey just three months later by mutual consent, however.
He was deemed surplus to requirements by Kenny Jackett after he took the hot seat at Fratton Park.Â
The skilful playmaker again linked up with Cook following his departure for Wigan Athletic, with the pair going on to land the League One title.Â
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