The 26-year-old was forced off during his side’s 3-0 win over Rotherham in the English Championship on Wednesday evening after slipping on the touchline and appearing to twist his ankle.
Lions boss Gary Rowett admitted afterwards that the injury “looks quite bad”, putting the former Hibs striker at risk of missing Scotland’s crucial Euro 2024 qualifier in Seville on October 12 and the friendly against France in Lille five days later.
Speaking after the game, Rowett said: “Nisbet I think – I’ve just watched it again down there – and there’s no one around him and he just almost turned his ankle. It looks quite bad but I don’t know until a couple of days [later and it] settles down. I’d imagine he’s going to be out for certainly the weekend and depending on scan results then we’ll see.
“It didn’t look great but again it’s impossible for me to say at this moment in time.”
Nisbet spent 10 months on the sidelines last year after rupturing his ACL while playing for Hibs. He recovered to score 10 goals in 21 appearances in the second half of last season before sealing a £2m transfer to Millwall in the summer, where he has scored two goals in eight appearances thus far.
The former Raith Rovers and Dunfermline front man was selected for the most recent Scotland camp which included the 3-0 win in Cyprus and the 3-1 defeat to England but was an unused substitute in both matches to remain on 10 caps.