Farrell was initially selected among the reserves, with Johnny Sexton chosen to work with Irish teammate Conor Murray in the halves, but was later ruled out with what the Lions described as a grade-one quadriceps injury.
"The step up to this tour is huge so you can't rest on the form you had and I knew it was going to be much harder". George Kruis 4. Maro Itoje 3.
With Billy Vunipola ruled out before the tour left Britain, losing another lynchpin in Farrell would be a bad blow to the Lions' prospects.
O'Mahony in Rotorua after being named captain. Dan Biggar fills Farrell's place on the bench. "Fortunately now, it's a hit out now this Saturday to look forward to".
"If we can get through the game getting those connections going again, getting back into the All Blacks' way, play with a bit of intensity and pace, with some accuracy, it will be good and obviously keeping an eye on the scoreboard", he said.
"They've been successful, guys around me have been successful and the amount of people I've learnt off - not just the captains who I've been involved with, but the other players I've been around as well".
"He reads the game well, has great game understanding, got all the skills, kick passes, he is fast so it is quite handy having a little player like him".
"We want to start the ball rolling and get away to a good start and whatever we do on Friday night we'll be able to improve on the following week", he said, adding that the focus first was to beat Samoa.
"Sam's well aware of the competition there at the moment", said Gatland.
The Maori All Blacks is a historic team in New Zealand which dates back to 1888, then called the New Zealand Natives, and were the first to wear the famous black jersey.
England fly-half Farrell suffered a minor thigh strain in training on Thursday, ruling him out of Saturday's tour match against an imposing Maori All Blacks side in Rotorua.
Coach Steve Hansen's named a team brimming with milestones, debuts and special occasions with full-back Ben Smith set to captain the All Blacks for the first time while all three Barrett brothers could play.
Ash Dixon captains the side from hooker as part of a pack which includes Elliot Dixon, Liam Messam and Akira Ioane. The Lions lost to the Maori when the teams last met in 2005.
While the Lions have seen individual All Blacks in action for Super Rugby teams so far on tour, Friday's match will give the tourists the first glimpse of a full All Blacks team and a better sense of the challenge awaiting them in the three-test series.
Every game of the tour will be shown on Sky Sports 1 at 8:35am BST (7.35pm local) and live on radio on Talk Sport.