Add that to the fact that Grant will be under considerable pressure to attack immediately (he will most likely not have time to organize his staff the way he would like) and McClellan is still going to be sulking like OTL, you get a recipe for a real sucker punch to Grant's career.
Grant might start off spectacularly, but Lee will stalemate him somewhere, and has the capability (unlike in 1864) to launch a wide ranging counter attack which could throw Grant off balance. Cue a long line of jealous subordinates blaming everything on Grant.
Grant could win the resulting campaign, but the poisonous atmosphere of intrigue and petty rivalry would really throw the Union army off balance, which is a boon to Lee and no one else. Most likely the army's confidence in Grant is shattered and Lincoln may then be forced to replace him.
Jackson still has a bit of free reign around early 1862 does he not. Yes, he is still under Lee's command, but rather independent in his approach. Might be a Grant/Jackson battle somewhere if possible.