Saints replacement Shane Geraghty was also injured late on with a suspected fractured eyesocket - an setback that will almost certainly seem him miss the start of the RBS 6 Nations.
But it is the injuries to Lawes and Ashton that will concern Johnson the most with both players emerging as key figures in the new look England side.
And Saints director of rugby Jim Mallinder hopes both will be back in contention by the time of England's opening game against Wales on February 4.
He said: "They've not broken anything. I know what you're all thinking but hopefully we'll get them back for next weekend.
"Both played on with their injuries. They are tough lads and I hope it is not too serious
"To lose two of our England men was not the best but I am not blaming that for the result."
But Johnson was given further food for thought by a host of impressive performances in both teams.
With lock David Attwood suspended, Leicester second row pairing George Skivington and Louis Deacon were immense in the boiler room while centre Anthony Allen was a livewire in the Tigers.