A farm vehicle hit the parapet of Bidford Bridge, in Bidford-on-Avon, Warwickshire, on Tuesday.
At the moment pedestrians and cyclists can still use the crossing, but town businesses said their summer trade will be hit by a total closure.
Warwickshire Council said a date when the repair work will be complete is "currently unknown".
Patrick Marshall, from The Bridge restaurant, said: "[People] can park on the other side and walk in, which is fine, but if they close it altogether while they do the work it could be devastating for us."
The council said it is doing "all that it can" to repair the bridge "as quickly as possible", according to Councillor Peter Butlin.
"[But] bearing in mind this is an old structure, it will have to be repaired using old methods, and they're not exactly quick," he said.
Matching stone for the 230ft (70m) long bridge will also have to be found before the repairs can be started, the council said.
Traders fear their businesses could be hit if a medieval bridge damaged in an accident has to be completely closed.