Zimmer did not give a timeline for Smith's return but acknowledged that the Vikings are preparing for the third-year tight end to miss the entire season.
"We knew it was probably going to be a little while anyway," Zimmer said. "Those are all part of things you have to plan for."
Sources told ESPN's Adam Schefter that Smith is facing a recovery timetable of 4-5 months. The Vikings placed Smith on the reserve/injured list later Wednesday.
Zimmer said Monday that Smith was injured during Minnesota's preseason finale at Kansas City, although it's difficult to discern how and when he suffered the injury. The tight end was on the field for only 11 plays, and there was no noticeable moment where he appeared to get hurt.
"Sometimes injuries are like that," Zimmer said. "You [don't know] until you get in there. Honestly, with Danielle [Hunter] last year, that was about as insignificant as you can imagine. Then you get in and X-ray and MRI and do all those things and find out it's more significant than you think."
Smith was primed for a breakout season at the time of his injury, having risen to the top of the Vikings' depth chart at tight end after the departure of Kyle Rudolph during free agency. With Rudolph injured during the final four games of the 2020 season, Smith stepped in and recorded three touchdowns on 20 targets, finishing the year with 30 catches for 365 yards and five scores.
"He's just a great football player, and there's no reason to think he wasn't going to have a great year, but he's got a lot of good football ahead of him," quarterback Kirk Cousins said. "I would expect, whenever he's back, the same great player that he was last year and that he was this August."
According to Zimmer, team doctor Dr. Christopher Larsen repaired Smith's injured meniscus instead of removing it, which could have led the tight end to develop an arthritic knee condition. Meniscus repair involves a longer recovery than a clean-up type procedure but will benefit the long-term health of Smith's knee.
On Tuesday, the Vikings made a move to mitigate the loss of Smith, acquiring veteran tight end Chris Herndon and a 2022 sixth-round pick from the New York Jets for a 2022 fourth-round selection. Minnesota also claimed former Jacksonville tight end Ben Ellefson on Wednesday.
After critiquing his team's tight end depth as "not very good," Zimmer said the Vikings would be looking to add additional players as they constructed the initial 53-man roster. Herndon spent three seasons with the Jets and found himself on the trading block after falling down the depth chart during training camp.
"He was a guy that was kind of pretty good in most areas," Zimmer said. "Decent blocker. Pretty good hands. Runs decent. Of all the tight ends we looked at, he was the one that we thought was the best option."
Herndon showed a lot of promise as a rookie in 2018, when he caught 39 passes for 502 yards and four touchdowns. Since then, the 2018 fourth-round pick has struggled to replicate that production. Herndon was suspended in 2019 and a DUI arrest and suffered a hamstring injury while working his way back onto the field. In his first game back in November 2019 against the New York Giants, Herndon cracked a rib and was placed on injured reserve.
Last season, Herndon struggled with drops and fumbles and transitioned into more of a blocking tight end. He bounced back toward the end of 2020, recording 31 catches for 287 yards and three touchdowns.
Minnesota will also lean on its No. 2 tight end, Tyler Conklin, in Smith's absence. Conklin returned to practice this week after dealing with a hamstring injury he classified as "minor."
Like Smith, the 2018 fifth-round pick had a strong showing at the end of the 2020 season. Conklin finished his third year with 19 catches for 194 yards and a touchdown and developed into a consistent blocker.
Even if Smith had been able to play this season, the Vikings didn't see specific responsibilities he would handle versus Conklin, which could bode well for this offense as it tries to compensate for the loss of its potential breakout star tight end.
"It's really not so much roles," Zimmer said. "It's really about, Conklin is a good player, and using these guys to their abilities. Not so much roles that each guy has. It may be we were planning on using Irv a little bit more on third down as a slot receiver. Probably won't do that now. Probably go with three wides. Some of those things we'll have to adapt and change."