LIONS boss Neil Harris admits he faces a "dilemma" on Saturday.
Aiden O'Brien's return from injury means Harris has 22 senior players to choose from ahead of the Championship clash with Barnsley.
If Harris includes O'Brien in his match-day squad, then one of Conor McLaughlin, Ryan Tunnicliffe, Jason Shackell, Fred Onyedima, Tom Elliott or Tim Cahill will miss out at Oakwell.
O'Brien had scored four goals in three games before injuring ankle ligaments against Leeds on January 20, two of them against Barnsley in the FA Cup third round in January that helped end a run of four successive defeats against the Tykes.
Harris has already had to leave Shane Ferguson, Ben Thompson and Harry Toffolo out of his last six squads as the Lions have continued their unbeaten run.
O'Brien played 64 minutes for Millwall's under-23s in their 1-1 draw with QPR at Loftus Road on Monday.
"He was seven weeks out and looked a little bit rusty, but it was good for him," Harris said. "He only had three training sessions last week so last Saturday was too soon. If he's okay then he is available for the weekend.
"It has and it hasn't been tough picking the squad. It hasn't been because the eighteen that I've gone with the last six games have earned the right to be there. But it's been tough on Ben and Shane and Harry. Aiden now comes into the mix and I have 22 fit players to choose from.
"It's a position I haven't been in at all really in the last three years. It's a healthy position but it is a dilemma going into the weekend.
"We have real attacking options at the stage of the season when we need to win games.
"We hadn't had a great run against Barnsley in the league, but obviously then beat them comprehensively in the cup."
Barnsley defeated Millwall 3-1 in the league at The Den earlier this season, but Jose Morais has since replaced Paul Heckingbottom as boss.
Harris said: "It's a different stage of the season and we're a different team now from earlier in the season.
"Barnsley are also a different team with a new manager. Everything goes out the window. All I'm interested in is we've won our last five away games in a row and want to make it six, and we're unbeaten in 11 league games and want to make that 12."
Image: Millwall FC