The visitors won the away leg in Amsterdam 2-1 thanks to goals from Karim Benzema and Marco Asensio.
"We gave it our all [against Barcelona] but were not decisive enough in the box," defender Dani Carvajal said. "We need to lay it all on the line [against Ajax]. They have had a good rest and we saw what they can do in the first leg. I hope the fans back us because we want to give them something to cheer."
Madrid's biggest threat against Barcelona was the speed and skills of Vinicius Junior. But the 18-year-old Brazilian failed to finish his scoring chances and got little help from the team's veterans. Benzema squandered his opportunities and Gareth Bale was jeered by segments of the crowd when substituted on Sunday.
The hosts are without suspended centre back Sergio Ramos, while Ajax have won their last three games by at least three goals.
"Goals come in patches," said Madrid manager Santiago Solari. "We'll keep plugging away. We can't let our teamwork, solidity and commitment slip."
Dortmund faltering, while PSG pick up the pace
As well as Real Madrid's match against Ajax tomorrow, there are three other Champions League ties this week.
Dortmund v Tottenham
After such a promising start under new coach Lucien Favre and an overhaul of the squad, Dortmund's season is unravelling.
Their 3-0 first-leg loss at Tottenham came after the side were knocked out of the German Cup and held to two draws in the Bundesliga.
Nothing has happened since to suggest Dortmund can recover tomorrow morning. They have only one win from seven games across
all competitions and have allowed Bayern Munich to draw level on points atop the Bundesliga after surrendering a nine-point lead.
"We're scoring goals against ourselves," Marco Reus said after his side's shock 2-1 loss at Augsburg on Saturday.
Harry Kane scored a late spot kick to salvage Tottenham a 1-1 draw on Sunday against Arsenal, who had a 91st-minute penalty saved.
PSG v Manchester United
United face the daunting task on Thursday of retrieving a 2-0 loss against a Paris Saint-Germain side led by the in-form Kylian Mbappe.
The 20-year-old France forward is helping PSG supporters forget all about Neymar. Mbappe scored in the first leg at Old Trafford and found the net twice at Caen on Sunday. He has now netted in all five domestic league games since the club lost Neymar and Edinson Cavani to injury.
"The coach said we had to win this game [against Caen] to prepare well for Manchester's visit and that's what we did," said Mbappe, who took his tally to 24 goals in 21 league games.
Mbappe will hope to profit from more open spaces as United chase the game without suspended midfielder Paul Pogba. PSG can also count on Cavani, who has resumed full training.
Porto v Roma
Roma and Porto are in crisis before they meet on Thursday, with the Italian visitors holding a slim 2-1 advantage.
Roma were beaten 3-0 by city rivals Lazio in their derby on Sunday to seriously damage their chances of qualifying for next season's Champions League.
"We really need to focus on Porto so we can get through to the quarter-finals," Roma captain Daniele De Rossi said. "If we do that, it will also change our vision of the season and our current form."
Porto are also reeling after losing 2-1 to Benfica on Sunday, a defeat which let their fierce rivals take the lead at the top of the Portuguese league.
- AP
