There is no requirement that if you do a rollover the entire amount must be rolled over. You can roll over as little or as much as you desire within 60 days of the distribution, but if less than the taxable amount is rolled over you'll be subject to income tax and potential early-distribution penalty on the taxable portion that is not rolled over.
As others have said, to roll over the entire distribution you would need to substitute other funds for the portion withheld for taxes.
If it has been more than 60 days since the distribution was made, you are stuck with the tax consequences of no longer being eligible to roll the distribution over.
