Seidl also conceded that the team has to find more performance as midfield rivals continue to improve their cars.
Norris started only 14th after a power unit sensor issue struck him in qualifying, obliging the team to go back to an older unit for the race, at a cost of some straightline performance.
At the second virtual safety car the team brought Norris in and double stacked him behind teammate Daniel Ricciardo.
Norris initially had to wait as the Australian had a slow stop due to an issue with the front right, and then his own tyres were not ready, and he lost further valuable seconds before he could resume.
Norris eventually finished 15th while Ricciardo missed the points in 11th after both drivers were told to manage their brakes in the latter part of the race.
"Obviously a disappointing day for us, a highly disappointing weekend for various reasons," said Seidl.
"Reliability issues, an operational issue today in the race, but also in terms of pace, and where we want to be.
"On the operation side during the pitstop today, we had a communications issue that was in the end snowballing into this issue that we have seen. We need to analyse and come back stronger."
Asked to explain what went wrong on the stop, he said: "Obviously, I don't want to go too much into detail, because it's something we have to review internally, as a team what went wrong there.
"Like I said before, in the end it was down to a communications issue within the team and that was causing the delays then."
Regarding the loss of pace in the latter stages, he said: "On both cars towards the end of the race, we had to manage some parameters, the brakes, and therefore it was not possible for Daniel to keep up the pace and to attack."
McLaren is still fourth in the championship but the team has scored just 19 points in the last five races and rivals are closing the gap, with Alpine logging 35 and Alfa Romeo 26 over the same stretch.
"Well of course if you look at the last two races, there's definitely a trend that we have been falling back compared to Alpine especially," said Seidl. "And also some other cars were showing signs of strong improvements like the Astons on Friday.
"So it is clear that we need to up our game in all areas, like I mentioned before, reliability, operations, performance, to make sure we stay in this battle for P4."
Seidl says no upgrades are in due in the short term: "Our idea is obviously to keep learning about the car and apply these learnings in order to extract more out of the current package.
"So far, there's no plans for any major upgrades for various reasons, including managing the cost cap as well."
On the plus side, Seidl believes that the power unit taken out of Norris's car before the race can be returned to the pool.
"What happened yesterday on Lando's PU was an issue with a sensor which was then causing a loss of power and a misfire, that's why we had to abort Q2.
"We did an analysis afterwards and in the garage we took the decision to switch on the power unit back to the other one which is in the pool, in the morning.
"We expect that the PU from [Saturday] can be repaired, and therefore at the moment there's no sign that we couldn't finish season with three power units."
