Kieran Tierney would be a welcome signing for Arsenal, but they need to bring in two centre-backs "immediately", says Charlie Nicholas.
Sky Sports News understands that a £15m bid for Tierney was rejected by Celtic over the weekend, but an improved offer from the Premier League side is expected in the coming days.
Nicholas praised the 22-year-old but was also cautious in his optimism. He told the Transfer Talk Podcast: "He's got real power, real pace going forward. He's turned into a very exciting full-back over the years and he's vastly improved.
"There is a question mark about his defensive frailties but then again he would be an improvement for Arsenal there's no doubt in my mind about that.
"Kieran Tierney is a fabulous footballer, but he's got a lot to learn defensively."
The Soccer Saturday pundit also questioned whether this is the area Unai Emery needs to strengthen most following a disappointing season.
Arsenal missed out on the Champions League for the second successive season after finishing fifth in the Premier League and losing heavily to Chelsea in the Europa League final.
When asked about potential signings, Nicholas said: "Two centre-backs. Immediately. They need a bit of pace - the guy who had pace was Laurent Koscielny but he kind of lost it about two to three seasons ago after the injuries.
"After that, I'm happy to add bonuses in midfield and potentially a winger or another striker but for Arsenal, ultimately it has to be two centre-backs.
"Arsenal under Emery have to be a unit. You have to get the defensive structure right. Nacho Monreal has actually done OK for Arsenal considering what his limitations truly were. He was a left-back, he was never a centre-back but he had to play there and that always told me that the reality was that Arsenal's big problem has been centre-backs.
"Hector Bellerin is still a bit away from fitness and we need cover but I'm really disappointed that we haven't got any youngsters coming through that are really threatening to take over.
"If we can get a left-back like Tierney then great but two centre-backs are the absolute key."