Paul Merson claims Arsenal's capture of Thomas Partey was the "number one" signing conducted by any Premier League club over the summer.
Partey moved to the Emirates Stadium from Atletico Madrid on Deadline Day after the Gunners met his £45million release clause.
The Ghanaian impressed on his debut - the 2-1 Europa League win over Rapid Vienna - and produced a masterful midfield display as Arsenal beat Manchester United in his second Premier League start.
Partey, 27, helped nullify a Red Devils side which put five past RB Leipzig only days earlier and his performance led to comparisons with Arsenal legend Patrick Vieira.
While Merson refused to get carried away by the 1-0 win at Old Trafford, he heaped praise on Partey for his "massive" display.
"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?'
Do you agree with Merson's declaration that Partey was the best summer signing? Have your say here.
"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."
Meanwhile, Merson wants to see Arsenal build on their win at United by putting together a run of victories.
He added: "Fair play to Arsenal, now you've got to hope they go and kick on.
"They've been unlucky, they probably shouldn't have got beaten by Leicester, at Manchester City they needed to score when they had a couple of chances, so for me they're not a million miles away from a very open league but they've got to kick on.
"There's no point getting beaten by Aston Villa next week now. You've got to go back to back.
"I said the other day 'Can I see Arsenal winning seven or eight on the trot?' No, I can't. But they need to start putting four and five wins together and if they can do that, who knows."
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