The 23-year-old injured his hamstring during Saturday's Scottish Cup quarter-final win over St Mirren.

It rules McKenna out of Scotland's Euro 2020 play-off semi-final at home to Israel on 26 March and a potential final five days later.

"The results of the scan was our biggest fear," McInnes said.

"It is a real blow. We anticipated him missing the next four games prior to the break, and as a consequence missing the Scotland games, but were hoping that he would be available for the semi-final.

"Scott has had his hamstring issues before, but this is a bit out of the norm. It is more the tendon coming off the bone, which is almost exactly like Funso Ojo's one against St Johnstone."

The midfielder was sidelined for three months from September.

"Unfortunately for us, the timescale makes it very tight for Scott being back playing for us this season again," McInnes said. "He is not ruled out, because everybody reacts differently to their rehabilitation, but that type of injury would normally keep you out the best part of three to four months, but Scott has been a good healer in the past, so you never know."

McInnes has added 19-year-old defender Jack Mackenzie, who has yet to make a first-team appearance, to his squad ahead of Wednesday's trip to face Kilmarnock in the Scottish Premiership and could move Mikey Devlin, who has been filling in at left-back in the absence of Greg Leigh, back to centre-half.

The manager said that Scotland midfielder Craig Bryson, who has only made 12 appearances since arriving from Derby County in the summer because of injury problems, will not be risked because of Rugby Park's artificial surface.

"It has been a season of significant injuries - not just boys out for two or three weeks," McInnes said. "We have to deal with it - the season is too important for us, we have got plenty to play for, we want to qualify for Europe, we want to get that third spot, we want to get to another Scottish Cup final."

Aberdeen sit fourth in the Premiership, one point behind Motherwell and are nine points and three places above Wednesday's hosts.

Centre-half Ash Taylor said of the amount of injuries the Dons have suffered: "The teams I've been in, I've never seen anything like it.

"It is really bad luck for Scott and for Scotland, but it is an opportunity for Mikey now."