If the US directed US or NATO troops to Ukraine that would be all that is needed.
The US could direct US troops to Ukraine, but not NATO ones; NATO has its own command structure that is not beholden to the US or its President, and the non-US members of NATO would have no reason to go along with any such US demand - they would, quite reasonably, ask which article of the North Atlantic Treaty was being invoked, and the answer would be "none".
If a Russian attack (on Poland or Germany, for example) caused the attacked nation to invoke Article 5, then at that point NATO command might decide that an attack on Russian forces in Russia, or Ukraine, or Belarus, was strategically justified. Or they might not. The US would get a say, but not a veto, nor a unilateral right to order any specific action.