Rangers striker Lawrence Shankland is set to miss three months through injury.
The Ibrox captain was forced off with a hamstring issue in the 1-0 win over Motherwell on Saturday.
Shankland had netted three times in the past two matches before suffering injury against Motherwell.
He had scored from the penalty spot against Aberdeen and then netted a double in the win over Falkirk.
However, Rangers are now set to face three months without having Shankland, with the Scotland striker expected to be out for at least 12 weeks.
Shankland's injury adds to the major knock suffered by Youssef Chermiti earlier in the season.
Rangers have Ryan Naderi, Bojan Miovski and new signing Kevin Kelsy as striking options ahead of the visit of St Mirren on Wednesday night.
"Yeah, well even more so now," McInnes said on Saturday when asked if Shankland's injury made the arrival of Kelsy from Portland Timbers all the more significant. "I think obviously if somebody had said to me walking in the door that Chermiti and Shankland were going to be injured with significant injuries, you know, you'd have felt the weight of that.
"And getting Kevin was such a good bit of business. The club deserves a lot of credit for that. And I thought he came on and got to the pitch of the game really well. I thought he... he's such a good mover, strong boy. There was obviously a temptation to bring him on after five minutes, but and much to the crowd's disappointment!
"But I had a conversation with Kevin yesterday. He said he felt a bit stiff and tight for the previous couple of days training and the fact that he travelled. So if he felt stiff and tight and not the right decision to start him, it's probably the right decision not to bring him on after five minutes and maybe exacerbate, put maybe more at risk the number nine position. And I was delighted for Ryan."
