Charlie Nicholas has pointed to the "biggest challenge" Shane Duffy will face as a Celtic player, while questioning his compatibility with one of Neil Lennon's favoured formations.
Duffy will join up with his new teammates this week after arriving in Glasgow following international duty.
He could make his Celtic debut against Ross County this weekend.
Nicholas reckons Duffy won't have any issues adapting to the domestic game in Scotland, and it's in Europe his defensive qualities will be put to the test.
He also said that the defender is at his best as part of a defensive four, and may not slot straight in to Lennon's favoured 3-5-2 formation.
He told the Express: "Shane Duffy's biggest challenges as a Celtic player will be in the Europa League games because very rarely do the team get challenged domestically, especially at Parkhead.
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"The Irishman was at his best as part of a back four at Brighton when he partnered Lewis Dunk.
"When Graham Potter took over and wanted a centre-half to pass the ball out of a three-man defence, Duffy found himself out of the team.
"He is an old-fashioned centre-back. That's what Celtic needed and now they have one.
"He is not the quickest but he will improve Celtic and give them a bit more steel."
