TONY Craig is closer to a first-team return after playing 45 minutes for the Elite Development Squad but he won’t be considered this weekend.
David Forde, Jimmy Abdou and Shane Ferguson have all returned from international duty to boost manager Neil Harris’ options.
Abdou and Ferguson were regulars in the starting XI before they missed two games while they were away with their countries.
Harris has shown already this season his consistency in retaining players who step in and perform well. O’Brien, who played on the left wing against Burton, scored the first goal.
Shaun Williams started in place of Abdou and complemented midfield partner Ben Thompson well, setting up the second goal for Lee Gregory.
Forde will be back on the bench instead of young goalkeeper Harry Girling.
Colchester looked dead and buried two weeks ago when their 2-1 defeat to Walsall left them 10 points of safety.
A 19-match winless streak from September to March makes them one of the favourites to get relegated to League Two. Three wins in March has given them renewed hope of avoiding the drop.
They came from behind to defeat Doncaster 4-1 last Friday before keeping just a second clean sheet in 29 games when they won 1-0 at Coventry on Easter Monday.
Goalkeeper Elliot Parish saved Jacob Murphy’s penalty against the Sky Blues, and he is looking forward to this weekend's clash.
“We’re confident and we do believe we can stay up,” Parish said. “Yes, it’s an uphill battle but you never know.
“We went on a great winning run at the start of the season so we’re more than capable of doing that. I believe that we’re a better team now than then.
“You can see the attitude of everyone – everyone is in it together and we’re a different side now, in terms of confidence.
“We’ll go into the Millwall game to win. They’re going well and so are we now, so bring it on.”
U’s boss Kevin Keen will give late fitness test to defenders Joe Edwards and Leo Chambers.
Millwall record against Colchester:
Won 25 Drawn 9 Lost 14
Form guide (Last five league):
Sky Bet odds:
Colchester 13/5 Draw 13/5 Millwall 1/1
Probable Millwall team: 4-4-2: Archer; Romeo, Webster, Beevers, J Martin; Taylor, Thompson, Williams, O’Brien; Morison, Gregory.
League One, November 21, 2015
Gregory 13, Ferguson 38, 62, Webster 81
Millwall: Archer; Cummings, Webster, Beevers, J Martin; O'Brien (Upson, 78), Saville, Williams, Ferguson (Onyedinma, 71); Gregory (L Martin, 83), Morison.
Colchester: Jones; Edwards, Eastman, Elokobi, Olufemi; Gilbey, Bonne, Massey, Moncur, Harriott; Sordell (Porter, 45).