Saint-Maximin came off in the sixth minute of stoppage time with Newcastle having used all their subs
Newcastle winger Allan Saint-Maximin will be out of action for a month due to a hamstring injury suffered in their victory over Southampton on Sunday.
The Frenchman left the pitch in the sixth minute of stoppage time and later left on crutches.
Newcastle have six matches up to the FA Cup third round fixture against either Rochdale or Boston on 4 January during the busy festive period.
On Sunday Bruce said he would "kick himself if he is out for a long time".
He admitted it is "the downside of playing somebody tired".
In Thursday's 2-0 win at Sheffield United, Saint-Maximin headed his first goal for the Magpies but was substituted in the 81st minute and left the field with a heavily strapped ankle.
The 22-year-old missed games against Norwich and Watford in August through injury, as well as the defeat by Liverpool in September.
Newcastle signed the Frenchman from Nice for £16.5m this summer and he has started nine of their 16 Premier League games, scoring and assisting once.
Source: bbc.com