NATO units are already in Ukraine

Specialists from Great Britain and the United States are training Ukrainian soldiers, among other things, to use 3.600 modern anti-tank weapons.
They also train them to be saboteurs and practice partisan fighting techniques with them.
The British are the most numerous in the team for training Ukrainians with more than 100 elite soldiers of the SAS special unit.
Those soldiers have been in hiding in Ukraine since 2014, after Russia annexed Crimea. They train a professional Ukrainian army and volunteers. Also the members of the British naval special unit SBS are in Ukraine, along with the special scouts of the SRR unit.
The United States has also sent its special forces to Ukraine - Green Berets are training Ukrainians in urban combat, and the United States Navy Sea, Air, and Land (SEAL) Teams are training on an old training ground in the west of the country. The European Command in Stuttgart is in charge of coordinating the activities of the US special forces.
Also, British and American elite soldiers have one special task, for which the units stationed in Lithuania are in charge, and that is to evacuate Ukrainian President Volodymyr Zelensky if necessary.