You need another table somewhere else (like another worksheet in the same file) that contains the Employee-Manager relationship. There you can reference the manager name from the existing list of employees, as you are currently doing in the original spreadsheet.
Once you have this done, you can use the VLOOKUP function to look up an employee's manager by the employee name. Given your example above, for contents for cell C2 would be
This is the syntax for VLOOKUP:
VLOOKUP(lookup_value, table_array, col_index_num, [range_lookup]). So in the example I just gave, lookup_value = employee cell, table_array = the new employee-manager table, and col_index_num = 2, since you want to return the 2nd column of the employee-manager table (the manager's name). This assumes you have placed the new employee-manager table on Sheet2 and started listing names in cell A2.
Even though your names list is "huge", you should be able to copy/paste it into the new sheet to save you typing time.
I hope I have explained this clearly enough. Additionally, for VLOOKUP to work right, the table_array has to be sorted in alphabetical order. Using Help on VLOOKUP will give you more information.