Valencia had only been introduced as a second-half substitute and helped set up Javier Hernandez's match-clinching goal, United's second.
However, the South American was injured in the build-up and was immediately taken off by manager Ferguson. Ashley Young scored the first goal.
"He has a hamstring injury unfortunately," Ferguson said of Valencia. "It will be four weeks."
Meanwhile, Ferguson said he was disappointed with his side's performance, but pleased with the result.
"I'm delighted with the scoreline," he said.
"It was a very ordinary performance from us though. We didn't hit any great heights.
"We improved in the second half and probably deserved to win it in the end.
"I know Ajax can make it difficult with the system they play, but we didn't get any tempo into our game.
"It was not a brilliant performance, but good enough."
Despite that, Ferguson expects to progress to the next round, where they will face either Lokomotiv Moscow or Athletic Bilbao.
"It gives us a good opportunity, but we won't take it for granted," he said.
"But at home at Old Trafford we have a good opportunity to get through."
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Comments
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to be honest it was poor stuff. Utd look like a team not good enough for the bigger stage.
Cannot believe thet he risked Valencia who has been the most influential player over recent weeks for a second hal bit part in a nothing game.
That is a pity !
Bad luck with Valencia who imo was hitting top form hope he is OK.
Not a classic performance for the reds but it is 2 away goals. Watched the the City match and it was a better game and Porto were a far better team that Ajax. We really have to up our performance if trophies are to be forth coming.
Noooooooooooo! We're dooooooomed !
It just shows how far SAF has lost his managerial nuance. Even yesterday, I mentioned in many a post that Utd must not field their s0-called better players for fear of injury. Europa League can't be a priority, there's no added glory in officially being the 33rd best team in Europe, assuming of course, they win it. Now the only guy apart from Rooney who seemed to show some quality is injured? Why was he introduced so late in the game? I am sorry but SAF clearly lost it. He's now just clueless. Without Valencia, there probably wouldn't be even a semblance of a challenge to City in the league. A poor team just got poorer. Sigh.
Get well soon Tony V. Your recent form has been outstanding. I am sure that we will miss your contribution significantly.
Since his return from injury, for the first time in three years Valencia has proven to be a very good player. Come back quickly to celebrate the 20th title
Didn't play with enough fluency especially our wing play. Nani was mostly out of sorts. Will take the result and look forward to a regrouped United (minus AV) next time out. Can't wait to see a hopefully filled out de Gea next season!
To be frank, was a little disappointed by de Boer's side, lots of promising young players....i'm sure their time will come if they can keep their key players together.
A huge loss, Valencia has been our best player this year terrifying defences. United really are cursed with injuries this season, when will it end?
Valencia turned on the overdrive to reach that ball, unfortunately too much for his hamstring. We will mis him badly, his form this season has been even better than Ronaldo in many ways. Our best winger since Andre!!!
It was a good solid performance. Wining 0-2 away is always a good result. A number of players had a run out which will do them good.
Handy to have Nani or Young available to step in for Tony V... gives them a chance to try and make a stake for that role. Good for competition and game time in prep for the end of the season.
Not surprised by the performance TBH, Jones and Fab haven't played much, Cleverley was v rusty and couldn't get into the game and Hernandez doesn't offer much more than poaching - so it was never going to be a total football experience, more of a case of getting minutes in the legs of the squad against what we thought would be, and what turned out to be, a poor Ajax team.
A real shame for Valencia. He was playing really well and we will miss his contribution both in attack and in defence. A great opportunity for Nani to get back to that form he was showing before he was injured.
Massive blow arguably our best player currently Nani and Young need to step up how many more effing injuries are we going to have.
After last night you can see why we went out of the CHAMPIONS LEAGUE.Fergie gets his tactics wrong again.
To slow allowing a poor team to come onto us,De GEA made a great save one he would not have had to make.
Very sad about Valencia he is our best forward and defender at the moment.United will come unstook if they keep playing dangerous balls back to the keeper top sides need to be forced well up the pitch.sitting back in our own half will eventually spell trouble.Why was BERBATOV left behind as both Nani and Rooney were pathetic.
Valencia is one of the best player for us so far this season. He should be wearing the number 7 shirt. Hope he will be back after 4 weeks.
Good professional performance and superb away result. Still worried about Rooney's on/off performances though. Yesterday he just was never in the game.
Valencia our player of the season so far. massive blow for him to be out for 4 weeks.
Hopefully he'll be back picking up his form where he left off
It's huge loss.. He was immense on the right wing and defenders had nightmare to handle him.. Will be sorely missed his services!