According to the rules the injury cap is a separate 400K not the amount the injured player takes up. so basically if a marquee player gets injured you can't spend that wage if it is over 400K on someone else.
***Injury Replacements: Where a player is injured for 12 weeks or more the Club may apply to the Salary Cap Manager to recruit a replacement player. The replacement player will then be permitted to remain with the Club for the rest of the season. The replacement player's salary for the duration of the dispensation shall sit in a separate £400,000 cap containing the Club's other injury replacements. The Club may not exceed the £400,000 during the salary cap year. In the event that a player is injured and that injury crosses over two Salary Cap years the injured player must be injured for at least the first eight weeks at the start of the year, counted from 1 August.
The application must be made while the player is still injured and the replacement player must be:
•The same position as the injured player; not be deemed to be more experienced than the injured player (i.e. same or less);
•Given a salary which is either comparable or lower than the injured player.
When determining these factors the Salary Cap Manager shall consider the general state of the market for availability of players in that position, the timing of when the cover is required, and any other relevant factors.
So assuming Bath isn't going to spend anymore on injury cover, it means that 400,000K has got to cover the transfer fee (potentially) and the wage.