Newcastle's 4-2 FA Cup third round replay win at Blackburn came at a cost after they lost both Ciaran Clark and Jamaal Lascelles to injury.
Magpies boss Rafael Benitez made eight changes to the side which started in Saturday's 2-1 Premier League defeat at Chelsea.
However, Newcastle made a flying start at Ewood Park when Sean Longstaff scored after a minute with a deflected 25-yard shot, before Callum Roberts added the second on 22 minutes.
But, Rovers came back to level before the break thanks to Adam Armstrong's finish and Darragh Lenihan's powerful header.
The tie went to extra-time where Joselu and Ayoze Perez were on the scoresheet to seal a first win in seven matches in all competitions for Newcastle and booked them a fourth-round home tie against Watford.
But the loss of defensive duo Clark and Lascelles because of respective knee and hamstring problems is something Benitez can ill-afford with his side 18th in the Premier League and facing fellow strugglers Cardiff City at St James' Park on Saturday.
The Spanish tactician said: "We need to wait until Wednesday and see how they are, and if they cannot play then we will bring another one in. Jamaal Lascelles was limping because he felt something.
"When it is a hamstring, you never know."
On the win, Benitez added: "The young players did well, and the main thing was that everybody was giving 100% for each other and that is what we did.
"The commitment and the team effort is there and you have to be pleased and carry confidence forward when you score four goals. The main thing is to get three points against Cardiff."
Newcastle, who are 66/1 to win the FA Cup this season, are 10/11 to defeat Cardiff in the league this weekend, while the Bluebirds are priced at 10/3 and the draw is 23/10.
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