The Croatian opened the scoring but then limped out of the action after 32 minutes after injuring himself when making a challenge on Albion defender Gonzalo Jara.
Modric left the ground on crutches and is to undergo a scan after suffering a blow on the same bone which was broken last season and sidelined him for several months.
It was not the news Redknapp was hoping for before Tuesday's Champions League clash away to Werder Bremen.
Redknapp said: "He is going for a scan but he broke the bone in his leg last year against Birmingham and he got a kick on the same spot today.
"He is in a lot of pain today. It doesn't look great.
"We will wait and see what the scan says but it certainly doesn't look very good.
"We don't know yet if the leg is broken again. If he has got another break, you are talking months again.
"I am just hoping it is good news for a change. Defoe has had the ankle operation and he is out for 12 weeks and Dawson is another one."