He said: “Although it was 1-1 at half-time I felt we’d missed an opportunity and I saw signs that we weren’t really defending properly.
“The second goal was a clear indication of that and then the sending off obviously put us on the back foot.
“I’m really disappointed. However, the only saving grace is that there’s not three points lost on this game today.”
Neil Harris heaped praise on striker Aiden O’Brien after he scored twice in Millwall’s 4-1 win.
Barnsley were the better side in the first half-an-hour, and took the lead in the 11th minute, when Adam Hammill’s floated cross sailed over the head of Tom Bradshaw, but fell kindly to Brad Potts to lash home the opening goal.
Goals either side of half-time from O’Brien (2) and Ben Thompson saw Millwall take control of the tie and when Joe Williams was sent off for a vicious, late challenge on Jed Wallace, it was all but over for Barnsley before Fred Onyedinma rounded off the game with a late goal.
Harris believes that O’Brien’s brace will do wonders for the striker’s confidence. “I was delighted for Aiden. He’s been a goal threat in every game he’s played, he’s been close and denied from goal-line clearances and the woodwork, but to get a couple now will do his confidence a world of good.”
Harris says the focus is on staying in the Championship, but he would love a run in the cup.
Millwall: Martin, McLaughlin, Hutchinson, Cooper, Craig, Wallace (Twardek 75), Thompson, Williams, Onyedinma (Romeo 85), Morison (Gregory 72), O’Brien. Unused substitutes: Archer, Meredith, Tunnicliffe, Saville.
Barnsley: Davies, Cavare, Pinnock, Lindsay, McCarthy, Gardner, Joe Williams, Potts (Mallan 73), Isgrove (Moncur 64), Bradshaw, Hammill (Thiam 64). Unused substitutes: Townsend, Brown, Ben Williams, Smith.
Referee: D Bond (Lancashire).