Muller joined Barnsley from Norwegian side Lillestrom
Barnsley manager Simon Davey refused to criticise goalkeeper Heinz Muller despite his two horrendous errors in the 3-0 defeat to Ipswich Town.
The German's poor clearance kick lead to Ipswich's first and his failure to collect a weak Ivan Campo long-range shot gave the host's their third.
"I signed Heinz 18 months ago and he's never made a mistake in all that time," Davey told BBC Radio Sheffield.
"I won't criticise him because he's been magnificent for me."
Davy added: "He's man enough to hold his hand up to the mistakes and he's apologised to the boys."
Meanwhile, the Barnsley boss refused to comment on the future of Brian Howard.
Sheffield United had declared they had signed the midfielder, but a Barnsley statement has since refuted the claim.
The Reds have said the terms of the Blades' offer was not acceptable and instead another un-named club had come in with an acceptable bid.
"I don't know what is happening and I'd prefer not to comment on it," said Davey.
"I haven't a clue who he will join and as far as I'm concerned I have to concentrate on the players on the pitch and in my dressing room and that's what I'm going to do."
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