Unai Emery believes Manchester United are a "different team" with Ole Gunnar Solskjaer at the helm.
The Red Devils travel to face Emery's Arsenal on Friday night in the pick of the FA Cup fourth round ties.
The two sides shared a 2-2 Premier League draw at Old Trafford last month, shortly before Jose Mourinho was sacked as United boss.
Since then, Solskjaer has overseen seven wins out of seven and closed an eight-point gap on the Gunners in the race for the top four.
Mourinho lost his job following a 3-1 loss at Liverpool, with his style and philosophy widely criticised by United supporters.
But Emery feels Arsenal will face a much-changed outfit when they meet for a place in round five, with Solskjaer's approach appearing to refresh his players.
"It's a different team," he said.
"It's the same players but they're playing with a big performance now. I was watching their last matches and and each player has a lot of confidence, with big performances, and now they are very dangerous.
After @DCFCOfficial's brilliant comeback last night, all of the Fourth Round ties are confirmed. Are there any more big upsets to come? 👀 pic.twitter.com/V7ZK9wXb6k — The Emirates FA Cup (@EmiratesFACup) January 17, 2019
"For us, it's good because it's a very big test for us in this competition."
Arsenal's preparation for the week included a team-building paintball trip but there have been a few difficult days for the club.
It was announced on Monday that head of recruitment Sven Mislintat will leave next month having only been at Arsenal for a little over a year.
A day later and it was confirmed Hector Bellerin could miss up to nine months after suffering ruptured knee ligaments in the 2-0 win over Chelsea on Saturday.
Yesterday we started the week in a different way... Great #teambuilding... We are the @Arsenal ! pic.twitter.com/ex5ORPwGuN — Unai Emery (@UnaiEmery_) January 22, 2019
Emery was asked about both issues and believes Arsenal should take their time in appointing a successor to Mislintat, with Roma's director of football Monchi linked to a similar role in north London.
Asked if it was important to swiftly find a replacement for Mislintat, Emery replied: "Maybe if you do this recruitment very quickly, maybe you can make a mistake.
"I think it's better to do without that, being calm and also finding the best possible person to do this job like we want.
"I think the club is working on that. It's not my issue. I can say the same: if the club decided to sign one sporting director, then I will agree with them."
❤️ @HectorBellerin pic.twitter.com/5OiqyL2c5G — Arsenal FC (@Arsenal) January 22, 2019
Bellerin will undergo surgery on his knee injury and could miss the start of next season given the severity of the damage to his knee.
"We were sad and we are sad now after his injury," Emery added.
"I think this year is very difficult for big injuries like Hector, Rob Holding and (Danny) Welbeck. But football is like that. We need to treat these injuries like a normality in football, but we are very sad for that.
"Hector Bellerin is very mature and I think he received this injury like a football professional can take and he is already preparing to come back stronger than before."
