Newcastle signed Wissa in a £55 million ($112.2 million) deal on transfer deadline day, also adding Stuttgart's Nick Woltemade in anticipation of Alexander Isak's British-record move to Liverpool.

Wissa scored 19 Premier League goals for Brentford last season, with none of them coming from the penalty spot. Only Mohamed Salah (20) netted more non-penalty goals.

However, the 29-year-old was substituted during DR Congo's FIFA World Cup qualifier against Senegal on Wednesday (AEST) after sustaining a knee issue, and he must now see a specialist.

Boss Eddie Howe has confirmed Wissa will miss Sunday's (AEST) Premier League match against Wolverhampton Wanderers, while his participation in Newcastle's opening UEFA Champions League game against Barcelona is also uncertain.

"He won't make this game. I saw him for the first time yesterday," Howe said.

"He's feeling the effects of the injury he sustained just before he came off, so we are going to have to see how he is."

Asked if the forward could be available to face the Blaugrana at St James' Park on Friday (AEST), Howe added: "I don't know. I think he will go away for tests and maybe see a specialist to see. Unfortunately, I can't give you that update."

Howe also confirmed Ramsey, who arrived from Aston Villa in a £43 million ($87.7 million) deal last month, will be out until October after suffering an ankle injury in a goalless draw with Leeds United prior to the international break.

Newcastle's transfer window was dominated by speculation over Isak's future and the Magpies' need to source attacking reinforcements, and Howe is relieved it is now over.

"It was a challenging window but also a successful window, hopefully. Time will be the best judgement of that," he said.

"Our clear focus now is just football. There are no off-the-pitch distractions, and we are looking forward to that. I am excited by the possibilities of this squad."

The fact that Wissa only had one year remaining on his contract with Brentford led to suggestions Newcastle overpaid for the attacker, but Howe is simply glad to have him.

"An established Premier League goalscorer is very difficult to find and difficult to recruit," Howe said.

"He has got great qualities, he had an outstanding year, not just last year but also the season before, so he's got a history of scoring goals.

"He's done well against us and been a difficult player for us to handle when we have come up against him. I really admire the journey he has been on to get to this point because it has not been easy for him."