Bert Lance's abrupt departure as director of the Office of Managemnt and Budget leaves the key agency leaderless just as it is beginning its most critical period - the annual fall review in which budgetmakers begin the cutting process that produces the January spending plan.

As Lance was going through his final trials last weekend, budget officials were getting ready to begin the long process of assembling fiscal 1979 recommendations from the various departments and matching them against the administration's overall budget targets.

The budgetmaking process will go on, as it does every year no matter who is in office. Day-to-day direction of the OMB is expected to continue under James T. McIntyre Jr., Lance's deputy, and W. Bowman Cutter, the executive associate director for budget matters.

What will be conspicuously missing, however, is what one official described as the "most valuable asset an OMB director can bring" to the budgetmaking process - direct clout with the President in pushing through the difficult decisions needed to draw a viable budget.

Carter has said he wants to impose a ceiling on spending for fiscal 1979, to hold outlays at about this year's levels plus an adjustment for inflation. Observers say that goal would be difficult under any circumstances. Without an influential budget chief, some fear it could be lost.

This is also the year the President has ordered a government-wide shift to so-called zero-base budgeting, the complex budgetmaking process Carter and his men imported from Georgia and under, which no spending program is spared an annual review. Georgia officials say the new ZBB procedure, as it is called, often requires for-feeding of reluctant agencies. Without a strong budget director, some fear, that effort also could fall apart.

Then, too, this is the year that the budget director must become deeply involved in such important side issues as government reorganization and welfare reform - not to mention the economic policymaking role that the head of OMB has played in recent years.

To many analysts, it is here that Lance's departure may leave the biggest void. Whatever his expertise in the economic field, the burly former banker consistently was a conservative force in the administration's otherwise moderate-to-liberal economic councils.

Indeed, Lance was perceived by the financial community as business' link with the new Democratic administration. Whatever his resignation may do to salvage the White House politically, some observers worry that it could leave the President with the new problem of maintaining business confidence.

In the day-to-day operation of the budget-agency, Lance was not known as a nuts-and-bolts manager. Despite his sometimes unfortunate experience in banking, the gregarious former Georgia gubernatorial candidate was not comfortable with budgetary figures. Often he would mix them up in talking with reporters.

"Lance never really got into the nitty-gritty details" said one OBM official in a position to observe the operation. "He controlled the fundamentals: then McIntyre would organize everything and Lance would approve the results. Lance's major interest was in the kind of decisions that went to the President."

The budget process that is scheduled to begin in a few days will intensify gradually in the coming weeks, with final decisions slated to be made by the President in late November and early December. By early January, the budget document must be completed and sent to the printer.

This year, those decisions not only will involve spending and economic policies, but major proposals the President has said he will make on tax revision, overhaul of the welfare system and government reorganization. At the same time, regular requests for spending increases will have to be held down.

Moreover, when the budget does come out in January, the administration will have to sell its proposals to Congress. That, too, will take political skills that Lance, more than any other key Carter economic official, seemed to have.

Carter insisted yesterday that the budget process had moved along on schedule so far, even though Lance's time had been diverted almost entirely to his defense. "There has been absolutely no slippage in the schedule Bert and I have devised," the President said in his press conference.

Despite Lance's closeness to the President, no one is predicting that the budget process will break down because of his resignation. But there is little doubt that in the next few - and crucial - months of budgetmaking, Carter will miss his old friend.