While the visitors squandered the chance to finish in the playoffs on the last day of the regular campaign, Boro lost out in the playoff semi-finals to Coventry City.
Match preview
With Middlesbrough having gone from relegation candidates to promotion contenders under Michael Carrick, it was natural that Boro felt as low as they did when succumbing to Coventry in the playoffs.
The North-East outfit had been regarded as the favourites for promotion, but rather than be preparing for Premier League football, Carrick is now attempting to rebuild his squad.
Top-flight loan additions will inevitably follow once they become available, yet Carrick has already overseen nine signings at both senior and Under-21 level this summer.
Three of those are goalkeepers in Seny Dieng, Tom Glover and Jamie Jones, while Boro fans will be intrigued to see how the likes of Dutch defender Rav van den Berg and Australian winger Sam Silvera fare in the second tier of English football.
Just two wins have been posted from seven games during pre-season, although Boro did end their schedule with a 2-2 draw versus Auxerre last weekend.
As far as Gary Rowett is concerned, he is tasked with the job of lifting a Millwall side that let slip a lead over Blackburn Rovers on the final day of the last campaign to miss out on a playoff spot.
Millwall would have fancied their chances of upsetting the apple cart in the end-of-season shakeup, but they now have to find a way to compete in what is, on paper, a much tougher league this time around.
Rowett will be delighted with the acquisitions of the likes of left-back Joe Bryan and forward Kevin Nisbet, both players adding extra quality in those areas.
Former Belgium Under-21 international Casper de Norre may also have an important part to play, although Millwall head into their season opener on the back of a draw with Charlton Athletic and defeat to Dutch side Fortuna Sittard.
Middlesbrough form (pre-season friendlies):
Millwall form (pre-season friendlies):
Team News
Boro will have to make do without last season's top goalscorer Chuba Akpom, who has missed the entire pre-season campaign through injury.
The bulk of the team that started the draw with Auxerre should feature here, with Hayden Coulson in line to play at left-back.
Of the new additions, only Dieng and Morgan Rogers appear in line to start, while the returning Martin Payero will either be selected on the substitutes' bench or left out the squad.
Rowett has decisions to make regarding the make-up of his Millwall XI with each of Tom Bradshaw and Nisbet being selected in attack for the Fortuna Sittard friendly.
On this occasion, Zian Flemming may return to the side to replace one of the frontmen, while George Saville could feature in midfield versus his former club.
Middlesbrough possible starting lineup:
Dieng; McNair, Fry, Lenihan, Coulson; Barlaser, Hackney; Forss, Crooks, McGree; Rogers
Millwall possible starting lineup:
Long; Wallace, Hutchinson, Cooper; McNamara, B.Mitchell, Saville, Bryan; Watmore, Flemming; Nisbit
We say: Middlesbrough 2-1 Millwall
Even though this is the opening weekend, the pressure is on Boro to make a fast start. We feel that may not necessarily help them going forward, yet Carrick's side may just about edge an hard-fought contest at the weekend.