Arsenal triggered the £45million release clause of Thomas Partey on the deadline day and brought him to the Emirates from Atletico Madrid.
The signing of the Ghana international is proving to be pivotal for Mikel Arteta and his team.
His performance against Rapid Vienna and Manchester United led to comparisons with Gunners legend Patrick Vieira.
“It was a good away performance, it wasn’t great,” Gunners great Merson told Sky Sports.
“I wouldn’t get carried away and say ‘Oh my God it’s a turning point’.
“They beat a terribly, terribly poor Manchester United team on the day. They’ve done a job, we’ve seen them lose those games over the last five or six years.
“But I think Partey’s a massive, massive difference. I mean, what a signing.
“When you talk about signings in the summer, he’s got to be number one. I cannot believe, without being horrible here, that Arsenal got him.
“I don’t mean that in a horrible way, they’re all crying out for it – I know Manchester United are top-heavy and they’ve got Fred, (Scott) McTominay and (Nemanja) Matic, but they haven’t got a Partey.
” Manchester City too, Fernandinho’s coming towards the end, Rodri hasn’t really pulled up any trees. You’re thinking ‘Wow, who didn’t see this?’
“When you’re talking about holding midfield players and you want discipline, there’s no better place for discipline than Atletico Madrid.
“They play the ultimate disciplined way. They can go away and shut up shop, then other days they can go and attack teams. He’s a massive part of that.”
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