It is the latest in a string of cruel injury blows for Onyeama-Christie, who was forced to miss the end of last season with a broken arm.
The injury also saw him miss Scotland’s summer internationals, having impressed in the Six Nations for Gregor Townsend’s side.
The versatile flanker, who can also play No8 or back-row, also suffered a broken forearm 12 months before that, missing the conclusion to the 2023 campaign, making this a third serious injury in just 18 months.
He had begun this campaign in style, impressing in all three matches and scoring two tries as Saracens won their opening game of the season at Gloucester.
“It doesn’t look great, I don’t think,” McCall said. “We’re talking maybe a fracture/dislocation of his ankle. Just gutted for him, everybody is gutted for him.
“He’s had a really tough few years, two broken arms. For that to happen is just horrendous for him. All our thoughts are with him.”
McCall insisted Saracens would support Onyeama-Christie and added: “He’s put so much work in post [after] that second broken arm – a cruel end to last season.
“He was the first player at the club to come back over the summer. He has played brilliantly for us since the start of the season so for that to happen is horrendous.
“We’ll look after him and get him back.”
Onyeama-Christie made his international debut off the bench in the 2022 Six Nations but injuries have limited him to just eight caps so far.
It seems unlikely he will recover in time for Scotland’s autumn internationals, which start with the visit of Fiji to Murrayfield on 2 November, with world champions South Africa to follow the following weekend.
Scotland begin their 2025 Six Nations at home to Italy on 1 February.