The striker did not reappear after half-time of the 3-1 home win over Birmingham City and although Boro boss Tony Mowbray doesn't think the problem is too bad, he understands that his attacking options are threadbare.
"We're looking hard to try to find some competition in the forward areas of the pitch and there's an urgency to do that with Scott's injury," he said. "We had two fit strikers at the club and we've got one now.
"I don't think it's a bad one, I'm hoping it's only a slight strain and it was easy for me to make a tactical chance and match them in midfield."
He added: "If the right offer comes in for the right player and we can utilise the money to strengthen other areas of the pitch then we will look to do that for the benefit of the team.
"We're in a position where we don't have to sell anyone but there's no ready cash to, say, buy another striker."
The likely absence has put McDonald in doubt for Australia's opening 2014 World Cup qualifiers against Thailand and Saudi Arabia in Brisbane and Riyadh respectively in early September.