Assuming the pre-acquisition plan that was not completed was related to finances, I think, the senior management team of the acquirer should immediately access the need for financing and to run operations smoothly and to keep vendor relations in good standing. Budget should be prepared along with weekly forecast that should be reviewed on daily basis.
The management must objectively assess whether the acquisition will add value or not, by comparing two scenarios:
1) Integrate target
2) Sell target
The analysis includes strategic, financial as well as cultural considerations. Doing the financial analysis, sunk cost must be ignored. If they conclude that the acquisition completed is not reasonable (in the sense that option 1 is less attractive than option 2), it should sell the acquired company.