CHELSEA boss Thomas Tuchel said his side failed to show "enough courage" in their narrow FA Cup win over Championship side Barnsley.
Tammy Abraham scored the winner with his side's only attempt on target to book a quarter-final home tie with Sheffield United.
Abraham was forced to make a headed goal-line clearance towards the end to deny Barnsley substitute Michael Sollbauer an equaliser.
"We scored the goal and I had the feeling that they increased the risk, but we stepped down in the little decisions," added Tuchel, who made 10 changes to his side.
"Not enough courage and it was too deep for a long, long time. We clearly can play better, so yes I expected more but not in terms that I am frustrated.
"We can do better but I don't want to be too hard on the players."
Hard-working Barnsley were the better team in the first half with Kepa Arrizabalaga blocking Callum Brittain's attempt at point-blank range.
The hosts claimed Abraham was offside for his goal but the England forward just held his run to ensure he was onside to finish from close range.
Chelsea are now unbeaten in five games since Tuchel took charge in January and will entertain Sheffield United in the sixth round on the weekend of 20-21 March.
Chelsea's fringe players given chance to impress new boss
Chelsea were languishing ninth in the Premier League when Tuchel took charge but are now one point off the top four and two wins from a fourth FA Cup final in five seasons.
This was a chance for Tuchel to take a first close look at his fringe players but they largely struggled to impress - and failed to register an attempt on target during a disjointed first half.
Kepa, demoted by previous boss Frank Lampard in favour of Edouard Mendy, kept his side on level terms with a fine save to keep out Brittain, who should have scored after Alex Mowatt's cross was flicked into his path.
In addition to Kepa, Emerson Palmieri, Kurt Zouma and 19-year-old midfielder Billy Gilmour were also appearing for the first time under Tuchel, while Christian Pulisic and N'Golo Kante made first starts since the German came in.
Brittain went close again after firing through a sea of legs, while Chelsea's only meaningful first-half attack ended up with Pulisic sending Callum Hudson-Odoi's cross wide.
The winning goal came from a lovely move with Gilmour at its heart. The ball eventually came to James who squared to Abraham for his 12th goal of the season.
Even then Chelsea had to endure a nervous ending against an impressive Barnsley side full of energy.
Sollbauer went close to equalising only for Abraham to clear his goal bound attempt as Chelsea lived dangerously.
'Proud of the performance' - what they said
Barnsley boss Valerien Ismael: "It's exactly the game we wanted, we made a great performance. We could see how far we can bring our principles, to make Chelsea make mistakes, we did it perfect in the first half but we had to score.
"It was a change in the second half, Chelsea made a change of shape and had more control. We were punished for one moment which was a shame. But we kept fighting to the end, we are proud of the performance."
Chelsea boss Tuchel on playing Sheffield United in the quarter-finals: "It's going to be a tough one. They are a very physical team that presses high. They are organised but I am happy to have a home game. This is what we wanted."
Chelsea in more FA Cup quarters than any other side this century
Since - and including - the year 2000, Chelsea have reached the FA Cup quarter-final more times than any other side, with victory against Barnsley ensuring their 15th appearance in the last eight in that time.
Chelsea's Tammy Abraham has been involved in nine goals in his last seven starts in the FA Cup (6 goals, 3 assists).
Since the start of last season, Abraham has scored 30 goals for Chelsea in all competitions, at least 11 more than any other player at the club.
No opposition player has scored against Chelsea in 450 minutes of action under manager Thomas Tuchel, with the Blues' only goal conceded under the German coming via an own goal against Sheffield United.
Barnsley return to Championship action away at Brentford on Sunday (13:00 GMT), while Chelsea host Newcastle United in the Premier League on Monday (20:00).
– Feb. 12, 2021 @ 08:49 GMT /