In the first half, Rashford was Manchester United's standout player, scoring the team's first goal and setting up Rasmus Hojlund as Ten Hag's squad quickly took a 2-0 lead.
Speaking on News about Rashford's first-half performance, former Wales striker Robert Earnshaw said: "Marcus Rashford has been excellent.
"Every time he gets the ball on the left he is terrifying Porto. He looks the most dangerous player on the pitch for Man Utd."
However, United were pegged back before the break as Porto pulled a goal back through Pepe and Samu Omorodion equalised before half-time.
TrendingRashford didn't re-emerge for the second half and was replaced by Alejandro Garnacho, with speculation mounting that he must have picked up an injury. However, after the game Ten Hag explained the substitution was all part of the plan as he looked ahead to the Super Sunday clash with Aston Villa, live on .
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cookies for this session only. Enable CookiesAllow Cookies Once"We have to rotate," Ten Hag said.
"Garnacho, we didn't start him, but he had a great game not only Sunday but the whole season.
"We go quickly with a turnaround to Villa and they have had a day longer to recover and we have an away game."
In his post-match press conference, Ten Hag added: "Yeah, but (replacement Alejandro) Garnacho was my best player on Sunday by far and also in many other games," he said.
"I think in the season, as I assess it so far, he is bringing us so far the offensive threat by creating chances, assists, also scoring.
"Garnacho we also have to play as well and we have two very good players over that side."
Asked if the decision had anything to do with Rashford's role in Porto's first goal, Ten Hag said: "As I say, I have to watch it back and I think over the left side definitely we didn't defend well tonight.
"Marcus also played a part in this but, as I say, it had to do with Garnacho and nothing against Rashy."
Porto struck again soon after the restart through Omorodion's second of the game and United, who were reduced to 10 men with nine minutes left when captain Bruno Fernandes was sent off for the second successive game after collecting a second yellow card for a high boot, looked to be heading for another damaging defeat.
However, Ten Hag's side dug in to salvage a draw when substitute Harry Maguire headed in from a corner during stoppage time.
Despite the point, United, who remain in the knockout play-off spots in 21st after a second draw, are winless in four games and the nature of this performance only adds to the pressure on Ten Hag ahead of the tough trip to Aston Villa on Sunday.
Pressed on his future, the Dutchman remained defiant, saying: "We will get there. Don't judge us in this moment, judge us at the end of the season.
"We are in the process, we will improve. We have two seasons now where we achieve finals, (so) just wait.
"We have to develop and progress this team. We will work and continue and we will fight.
"You see the spirit and the connection between the staff and the team, the players are together and want to achieve. We have that good mentality, but some defending parts we have to step up."
United have now conceded three or more times in a match on 24 occasions in all competitions since Ten Hag took over - more than any Premier League side has in that period.
This was the third time they have conceded three in a game this season alone.
However, goal scorer Maguire went on to back Ten Hag to use his experience to pull United through this difficult spell.
"I have played for this club for six years and I know how it works," Maguire said.
"When you go through a bad spell, players come under pressure, but also the manager does.
"He [Ten Hag] is experienced enough, he has been in the game long enough and has been at this club long enough, so I am sure he has learnt how to deal with it.
"It is part and parcel of playing for or managing this club, part of the privilege that you have."