Earlier on Tuesday, Boeing (BA) and Alaska Airlines (ALK) announced in the press release here that the carrier has placed an order to buy 23 more 737-9 airplanes.

Although Boeing shares closed lower yesterday, we think this news is worth going over again, because this was not just some incremental piece of information.

This new order builds on top of Alaska's original order to acquire 737-9s through a lease agreement. As part of the delivery schedule, Alaska will receive 13 planes in 2021; 30 in 2022; 13 in 2023; and 12 in 2024. In addition to this order of 68 planes, Alaska has the option to buy 52 more jets between 2023 and 2026.

Alaska Airlines said in its own press release here that these deliveries will largely replace Alaska's Airbus fleet and "moves the airline substantially toward a single, mainline fleet that's more efficient, profitable and environmentally friendly, and that will enhance the guest experience and support the company's growth." This is a sign that airlines are welcoming back Boeing aircraft, choosing the company over rival Airbus (EADSY) after what has been a very difficult stretch of time for the U.S.-based manufacturer.

This is the second big order by an airline for more 737 MAX jets this month. Earlier in December, Europe's biggest low-cost carrier Ryanair (RYAAY) ordered 75 more 737 MAX jets with a list price of $9 billion. And we still have not heard China's plans for a potential order to support its fast-growing aerospace industry and improve trade relations with the United States. Since it will be difficult to estimate what Boeing will earn over the next few years, because there are so many moving parts right now, we think free cash flow expectations is the most closely watched metric for stock price direction. What will drive free cash flow estimates higher is delivery expectations, and delivery expectations increase as more orders come in. This explains why deals like the ones with Alaska and Ryanair, and perhaps something in the future with China, are so important to the Boeing bull case.

Lastly, we want to point out that the 737 MAX is also less than one week away from returning to commercial flight, with American Airlines (AAL) scheduled to resume service on Dec. 29 through Jan. 4, with two flights a day (one round trip) from Miami to New York's LaGuardia Airport. This event may not represent a specific catalyst to look for, but it does represent a step in the right direction in terms of building back the confidence of air travelers.

All in, we believe the Alaska Airlines order further strengthens our investment case and we continue to view weakness as opportunities to scoop up additional shares, much like we did early Monday morning in our Alert here.

Action AlertsPLUS, which Cramer co-manages as a charitable trust, is long BA.